To narrow in an answer to the question, at least as I read the question, there are actually two questions:

1. Where (or in what context) is that concept used?
2. Where in earlier Christian writings is it used?

Within the Lutheran context, the authoritative collection of writings held to by many Lutherans for a long extent of time has been the Formula of Concord. In the article concerning good works, in the epitome, it states:

7. We believe, teach, and confess that for the retention of pure teaching concerning the righteousness of faith before God, it is particularly important to hold steadfastly to the particulae exclusivae, that is, the following expressions of the holy apostle Paul that completely separate the merit of Christ from our works and give honor to Christ alone. The holy apostle Paul writes, “by grace,” “without merit,” “apart from the law,” “apart from works,” “not through works,” etc. These expressions all mean nothing other than that we become righteous and receive salvation “alone through faith” in Christ. 8. We believe, teach, and confess that although the contrition that precedes justification and the good works that follow it do not belong in the article on justification before God, nevertheless, a person should not concoct a kind of faith that can exist and remain with and alongside an evil intention to sin and to act against the conscience. Instead, after a person has been justified by faith, there then exists a true, living “faith working through love” (Gal. 5[:6]). That means that good works always follow justifying faith and are certainly found with it, when it is a true and living faith. For faith is never alone but is always accompanied by love and hope.

Robert Kolb and Timothy J. Wengert, eds. The Book of Concord: The Confessions of the Evangelical Lutheran Church. Accordance electronic ed. (Minneapolis: Fortress Press, 2000), 496.

Notice the careful distinctions the authors set up:

• Salvation is received through faith alone
• But this faith never remains alone because good works always follow faith (et bona opera semper fidem justificantem sequi)

I hope that gives you a head start in answering the first question. I do suggest reading all of that article to get a better, fuller context though. The second question, as to where in Christian writings did this idea originate, in my judgment it's best to go back to the source and look at the biblical writings themselves. A good example of where the Reformers got the idea of good works being a fruit of faith is Ephesians 2:

“ But because of his great love for us, God, who is rich in mercy, made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved. And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast. For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do.” (Eph. 2:4–10 NIV11-GKE)

So, notice how Paul goes out of his way to let us know that salvation is completely done by God and from him:

• God's love, not our worthiness was the motivator (“διὰ τὴν πολλὴν ἀγάπην” (Ἐφεσίους 2·4 THGNT-T))
• This salvation is not from us (“καὶ τοῦτο οὐκ ἐξ ὑμῶν” (Ἐφεσίους 2·8 THGNT-T))
• it's a free gift from God (“θεοῦ τὸ δῶρον” (Ἐφεσίους 2·8 THGNT-T))
• Not by our works (“οὐκ ἐξ ἔργων” (Ἐφεσίους 2·9 THGNT-T))
• This salvation is received through faith (not because of faith) (“διὰ πίστεως” (Ἐφεσίους 2·8 THGNT-T))

the Bible is clear that, when it comes to salvation, our own good works have no part/place—not in whole, not even in part. But then look at where Paul goes. He let's us know then the proper context in which good works dwell and thrive in verse 10:

• We are God's created things (“αὐτοῦ γάρ ἐσμεν ποίημα” (Ἐφεσίους 2·10 THGNT-T))
• Why are we created? We are created for good works (“ἐπὶ ἔργοις ἀγαθοῖς” (Ἐφεσίους 2·10 THGNT-T))
• We are created to do (walk in) good works (“ἵνα ἐν αὐτοῖς περιπατήσωμεν.” (Ἐφεσίους 2·10 THGNT-T))

Passages in the Bible like this is where theologians have wrestled with the relationship between faith and good works. And, from these verses one can clearly see the concept that, as the receiving organ (ⲗⲏⲡⲧⲓⲕⲟⲛ ⲟⲣⲅⲁⲛⲟⲛ) faith alone justifies. But, it never remains alone. There is fruit that grows from faith.

OP: Where in Christian history did the saying, "Good works are the fruits of faith," originate?

While not specifically using that phrase, the concept that good fruit comes from Godly faith arises very early:

I have greatly rejoiced with you in our Lord Jesus Christ, because ye have followed the example339 of true love [as displayed by God], and have accompanied, as became you, those who were bound in chains, the fitting ornaments of saints, and which are indeed the diadems of the true elect of God and our Lord; and because the strong root of your faith, spoken of in days340 long gone by, endureth even until now, and bringeth forth fruit to our Lord Jesus Christ, who for our sins suffered even unto death, [but] “whom God raised from the dead, having loosed the bands of the grave.”341 “In whom, though now ye see Him not, ye believe, and believing, rejoice with joy unspeakable and full of glory;”342 into which joy many desire to enter, knowing that “by grace ye are saved, not of works,”343 but by the will of God through Jesus Christ. (Polycarp, Epistle to the Phillippians Chapter 1, emphasis mine)

From Polycarp, we may trace the same idea to his pupil Irenaeus:

4. And therefore throughout all time, man, having been moulded at the beginning by the hands of God, that is, of the Son and of the Spirit, is made after the image and likeness of God: the chaff, indeed, which is the apostasy, being cast away; but the wheat, that is, those who bring forth fruit to God in faith, being gathered into the barn. (Irenaeus, Against Heresies Book 5 Chapter XXVIII)

so likewise men, if they do truly progress by faith towards better things, and receive the Spirit of God, and bring forth the fruit thereof, shall be spiritual, as being planted in the paradise of God. Against Heresies Book 5 Chapter X

The Bible is clear enough that from faith comes good works (For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them. Eph. 2:10). This same idea is traced from the Bible to John's pupil Polycarp to his pupil Irenaeus.