I don't know which edition of the book you have. If it's the 30th anniversary edition, try page 145.

If not, the scene is near the end of Book I, right before Michael leanes for his meeting with Sollozzo.

Tom: "You shouldn't let that broken jaw influence you. McCluskey is a stupid man, and it was business, not personal."

Mike: "Tom, don't let anybody kid you. It's all personal, every bit of business. Every piece of shit every man has to eat every day of his life is personal. They call it business. OK. But it's personal as hell."