Where is the verse about people sinning because they know God will forgive them?

Ok, I just found it. I did get it a little mixed up with a different verse (My bad), but here's what I found:

The verse was: 1 John 3:6: No one who lives in him keeps on sinning. No one who continues to sin has either seen him or known him.

I got it mixed up with: 2 Peter 2:21: It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them.

I believe the passage you are looking for is Matthew 18:21-22.

Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him? till seven times?

Jesus saith unto him, I say not unto thee, Until seven times: but, Until seventy times seven.

If this is the measure of Jesus' expectation of us, then it can only be the measure he, himself, applies.

Sirach 6:5-9 Be not without fear about sin forgiven, and add not sin upon sin: 6 And say not: The mercy of the Lord is great, he will have mercy on the multitude of my sins. 7 For mercy and wrath quickly come from him, and his wrath looketh upon sinners. 8 Delay not to be converted to the Lord, and defer it not from day to day. 9 For his wrath shall come on a sudden, and in the time of vengeance he will destroy thee.

There is a verse in The Book of Mormon which closely matches your description. Perhaps the person whose sermon you remember this from was referring to that scripture.

2 Nephi 28:8

8 And there shall also be many which shall say: Eat, drink, and be merry; nevertheless, fear God—he will justify in committing a little sin; yea, lie a little, take the advantage of one because of his words, dig a pit for thy neighbor; there is no harm in this; and do all these things, for tomorrow we die; and if it so be that we are guilty, God will beat us with a few stripes, and at last we shall be saved in the kingdom of God.

The rest of the chapter does indicate that those who follow this path are in danger of hell, but doesn't specifically compare them to other sinners as better or worse.

That scripture references Isaiah 22:13-14 which also closely matches your description:

Isaiah 22:13-14

13 And behold joy and gladness, slaying oxen, and killing sheep, eating flesh, and drinking wine: let us eat and drink; for to morrow we shall die.

14 And it was revealed in mine ears by the Lord of hosts, Surely this iniquity shall not be purged from you till ye die, saith the Lord God of hosts.

So this is similar in saying eat, drink, and be merry, but verse fourteen could be interpreted as suggesting that the sin is as bad, or worse, than those that perform death-bed repentance.

James 4:17 If anyone, then, knows the good they ought to do and doesn’t do it, it is sin for them.(A)

When I search Google for "deliberately sinning, but God will forgive you" the one of the top results suggests these verses:

If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and are overcome, they are worse off at the end than they were at the beginning. It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them. (2 Peter 2:20-21, NIV)

If in spite of these things you do not accept my correction but continue to be hostile toward me, I myself will be hostile toward you and will afflict you for your sins seven times over. (Leviticus 26:23-24, NIV)

When I search Google for your exact wording "sinning and knowing it's wrong, but sinning anyway because you know God will forgive you" the one of the top results identifies Hebrews 10:26-31:

For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries. Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses. How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? For we know him who said, “Vengeance is mine; I will repay.” And again, “The Lord will judge his people.” It is a fearful thing to fall into the hands of the living God. (Hebrews 10:26-31, NIV)

There are a few verses in Romans that might be what you're thinking of:

Romans 3:8 ESV And why not do evil that good may come?—as some people slanderously charge us with saying. Their condemnation is just.

Romans 6:1 ESV What shall we say then? Are we to continue in sin that grace may abound?

Romans 6:15 ESV What then? Are we to sin because we are not under law but under grace? By no means!

Note: I could go in to what I believe these verses mean within their context but that would be outside the scope of the question. This question is for verse identification.

Jude 4 says:

I say this because some ungodly people have wormed their way into your churches saying that God's marvelous grace allows us to live immoral lives. The condemnation of such people was recorded long ago, for they have denied our only Master and Lord Jesus Christ."

New Living Translation.