Tom dislikes Gatsby from the start and ascertains quite early on that Gatsby might be involved in bootlegging. Eventually, Tom recognizes that there might be something between Daisy and Gatsby. In Chapter 7, Gatsby and Nick visit Tom and Daisy at their place. While Tom is on the phone with Myrtle, Daisy tells Gatsby she loves him. Tom sees this and of course is not happy about it.
She had told him that she loved him, and Tom Buchanan saw. He was astounded. His mouth opened a little, and he looked at Gatsby, and then back at Daisy as if he had just recognized her as some one he knew a long time ago.
This was something Daisy said. In Chapter 6, what initially set Tom off was when Gatsby told him that he knew Daisy. Knowing what we do about Tom, it is likely that Tom became immediately suspicious at this point.
In Chapter 7, Tom confronts Gatsby and Daisy and Gatsby fires back with what he's wanted to say for five years.
"Your wife doesn't love you," said Gatsby . "She's never loved you. She loves me."
From this point, the conversation escalates. Tom is furious and Gatsby has no choice but to completely divulge his love for Daisy. The argument heats up even more and this building intensity segues into the "death car" scene.