Can someone help me cite Luther's explanation of faith alone to Catholics

The text you are referring to does not exist in the form you have quoted. Rather what you have is a combination of some sentences from a lengthy letter by Luther in response to peevish criticisms that some Catholic sophists were making on Luther’s translation of the Bible into German. Luther seems to be in a ‘bad mood’ about it all, as He had used so much work doing something that his critics were unwilling to even try, and yet had to face their endless criticisms on subjects they knew little about (i.e. the German language) This does tend to make those that actually understand what they are talking about frustrated. Yet Luther’s anger does not seem completely excusable.

Note: I put in bold some of the fragments collected in your quote.

Here is a quote earlier in the letters showing why He is frustrated:

There is a saying, “He who builds along the road has many masters.”10 That is the way it is with me too. Those who have never even been able to speak properly, to say nothing of translating, have all at once become my masters and I must be the pupil of them all. If I were to have asked them how to put into German the first two words of Matthew’s Gospel, Liber Generationis,11 none of them would have been able to say Quack!12 And now they sit in judgment on my whole work! Fine fellows!

That is the way it was with St. Jerome too when he translated the Bible. Everybody was his master. He was the only one who was totally incompetent. And people who were not worthy to clean his shoes criticized the good man’s work.13 It takes a great deal of patience to do a good thing publicly, for the world always wants to be Master Know-it-all.14 It must always be putting the bit under the horse’s tail,15 criticizing everything but doing nothing itself. That is its nature; it cannot get away from it.

Later on in the letter He really starts to fume. This is where some of your sentences came from.

But to return to the matter in hand! If your papist wants to make so much fuss about the word sola (alone) tell him this, “Dr. Martin Luther will have it so, and says that a papist and an ass are the same thing.” Sic volo, sic jubeo; sit pro ratione voluntas.21 We are not going to be the pupils and disciples of the papists, but their masters and judges. For once, we too are going to be proud and brag with these blockheads; and as St. Paul boasts over against his mad raving saints [II Cor. 11:21ff.], so I shall boast over against these asses of mine. Are they doctors? So am I. Are they learned? So am I. Are they preachers? So am I. Are they theologians? So am I. Are they debaters? So am I. Are they philosophers? So am I. Are they dialecticians? So am I. Are they lecturers? So am I. Do they write books? So do I.

I will go further with my boasting. I can expound psalms and prophets; they cannot. I can translate; they cannot. I can read the Holy Scriptures; they cannot. I can pray; they cannot. And, to come down to their level, I can use their own dialectics22 and philosophy better than all of them put together; and besides I know for sure that none of them understands their Aristotle.23 If there is a single one among them all who correctly understands one proemium [preface] or chapter in Aristotle, I’ll eat my hat.24 I am not saying too much, for I have been trained and practiced from my youth up in all their science and am well aware how deep and broad it is. They are very well aware, too, that I can do everything they can. Yet these incurable fellows treat me as though I were a stranger to their field, who had just arrived this morning for the first time and had never before either seen or heard what they teach and know. So brilliantly do they parade about with their science, teaching me what I outgrew25 twenty years ago, that to all their blatting and shouting I have to sing, with the harlot, “I have known for seven years that horseshoe-nails are iron.”26

Let this be the answer to your first question. And please give these asses no other and no further answer to their useless braying27 about the word sola than simply this, “Luther will have it so, and says that he is a doctor above all the doctors of the whole papacy.” It shall stay at that! Henceforth I shall simply hold them in contempt, and have them held in contempt, so long as they are the kind of people—I should say, asses—that they are. There are shameless nincompoops among them who have never learned their own art of sophistry—like Dr. Schmidt28 and Doctor Snotty-Nose,29 and their likes—and who set themselves against me in this matter, which transcends not only sophistry, but (as St. Paul says [I Cor. 1:19–25]), all the world’s wisdom and understand understanding as well. Truly an ass need not sing much; he is already well known anyway by his ears.30

Although Luther did not really have to explain himself, he decides to anyway later in the letter:

To you and to our people, however, I shall show why I chose to use the word sola—though in Romans 3[:28] it was not sola, but solum or tantum that I used,31 so sharply do the asses look at my text! Nevertheless I have used sola fide elsewhere,32 and I want both: solum and sola. I have constantly tried, in translating, to produce a pure and clear German, and it has often happened that for two or three or four weeks we have searched and inquired for a single word and sometimes not found it even then. In translating Job,33 Master Philip,34 Aurogallus,35 and I labored so, that sometimes we scarcely handled three lines in four days. Now that it is translated and finished, everybody can read and criticize it. One now runs his eyes over three or four pages and does not stumble once—without realizing what boulders and clods had once lain there where he now goes along as over a smoothly-planed board. We had to sweat and toil there before we got those boulders and clods out of the way, so that one could go along so nicely. The plowing goes well when the field is cleared.36 But rooting out the woods and stumps, and getting the field ready—this is a job nobody wants. There is no such thing as earning the world’s thanks. Even God himself can earn no thanks, with the sun, indeed with heaven and earth, or with his own Son’s death. It simply is and remains world, in the devil’s name, because it just will not be anything else.

Here, in Romans 3[:28], I knew very well that the word solum is not in the Greek or Latin text; the papists did not have to teach me that. It is a fact that these four letters s o l a are not there. And these blockheads stare at them like cows at a new gate.37 At the same time they do not see that it conveys the sense of the text; it belongs there if the translation is to be clear and vigorous. I wanted to speak German, not Latin or Greek, since it was German I had undertaken to speak in the translation. But it is the nature of our German language that in speaking of two things, one of which is affirmed and the other denied, we use the word solum (allein)38 along with the word nicht [not] or kein [no]. For example, we say, “The farmer brings allein grain and kein money”; “No, really I have now nicht money, but allein grain”; “I have allein eaten and nicht yet drunk”; “Did you allein write it, and nicht read it over?” There are innumerable cases of this kind in daily use.

In all these phrases, this is the German usage, even though it is not the Latin or Greek usage. It is the nature of the German language to add the word allein in order that the word nicht or kein may be clearer and more complete. To be sure, I can also say, “The farmer brings grain and kein money,” but the words “kein money” do not sound as full and clear as if I were to say, word nicht or kein may be clearer and more complete. To be sure, I can also say, “The farmer brings grain and kein money,” but the words “kein money” do not sound as full and clear as if I were to say, “The farmer brings allein grain and kein money.” Here the word allein helps the word kein so much that it becomes a complete, clear German expression.

Finally later on the typical Luther mentions what really matters to him, the gospel. After all the reason why his critics were squabbling peevishly over a word is that they hated the grace of God that Luther offered to sinners. They wanted faith plus works, not free grace apart from works, like the Apostle Paul argues. There opposition of this word, in Luther’s view was not about words but about their opposition to Paul and Christ. So He argues against this anti-gospel stance as he always did. This was the heart of the reformation and Luther was its leader:

Tell me, further: What is the work by which we lay hold of Christ’s death and resurrection? It cannot be any external work, but only the eternal faith that is in the heart. Faith alone, indeed, all alone, without any works, lays hold of this death and resurrection when it is preached by the gospel. Why then this raging and raving, this making of heretics and burning them at the stake, when the matter itself at its very core is so clear and proves that faith alone lays hold of Christ’s death and resurrection, without any works, and that his death and resurrection [alone] are our life and our righteousness? Since, then, the fact itself is so obvious—that faith alone conveys, grasps, and imparts this life and righteousness—why should we not also say so? It is no heresy that faith alone lays hold on Christ, and gives life; and yet it must be heresy, if anyone mentions it. Are they not mad, foolish, and nonsensical? They admit that the thing is right, but brand the saying of it as wrong, though nothing can be both right and wrong at the same time.

Moreover I am not the only one, or even the first, to say that faith alone justifies. Ambrose said it before me, and Augustine and many others. And if a man is going to read St. Paul and understand him, he will have to say the same thing; he can say nothing else. Paul’s words are too strong; they admit of no works, none at all. Now if it is not a work, then it must be faith alone. What a fine, constructive, and inoffensive doctrine that would be, if people were taught that they could be saved by works, as well as faith! That would be as much as to say that it is not Christ’s death alone that takes away our sins, but that our works too have something to do with it. That would be a fine honor honoring of Christ’s death, to say that it is helped by our works, and that whatever it does our works can do too—so that we are his equal in strength and goodness! This is the very devil; he can never quit abusing the blood of Christ.

The matter itself in its very core, then, demands that we say, “Faith alone justifies.” And the nature of our German language also teaches us to express it that way. I have in addition the precedent of the holy fathers. And the danger of the people also compels it, so that they may not continue to hang upon works and wander away from faith and lose Christ, especially in these days, for they have been accustomed to works so long they have to be torn away from them by force. For these reasons it is not only right but also highly necessary to speak it out as plainly and fully as possible, “Faith alone saves, without works.” I am only sorry that I did not also add the words alle and aller, and say, “without any works of any laws,” so that it would have been expressed with perfect clarity. Therefore it will stay in my New Testament, and though all the papal asses go stark raving mad they shall not take it from me. (Luther’s Works, Volume 35, Page 182 - 198)