There are times the NT quotes the OT directly, and to give just one example: John 6:45 quoting Isaiah 54:13 and Jeremiah 31:33-34. We should get into the habit of asking for a Bible verse, but we also must keep in mind that the Scriptures sometimes don’t quote directly when referring to the other parts of Scriptures. For example, Paul says in 1 Corinthians 14:34, “Let your women keep silent in the churches… as the law also says.” Which law? Where in the OT can we find this teaching? You won’t find a single passage that teaches this in the OT. Reason? The OT [taken as a whole] teaches this. This is why sometimes it’s not possible to give a single reference. Apostle John understood this when he writes 1 John 1:5, “This is the message which we have heard from Him and declare to you, [that God is light and in Him is no darkness at all].” What John is doing here is summing up the essence of Christ’s message that they all have heard. If we take everything that Christ taught and summarize it into one short sentence, it would be 1 John 1:5! God is light and in Him is no darkness at all. If God is light, then we are darkness because we’re not God, which means we are sinful. And if there’s no darkness in Him, not even a small hint of it, then sinful men can never be near this God, and on and on it goes.
I can give plenty more examples, but let me get to the point: It’s not enough to just ask for a biblical passage (even though we should) because anyone can do that, even a fool and lunatic. We need to know our Bible, and this comes by daily reading, meditation and studying.