Sai's Betrayal!! (サイの裏切り!!, Sai no Uragiri!!) is chapter 303 of the original Naruto manga.
Sai claims to have no remorse about collaborating in an invasion of Konoha, but Sakura reminds Sai that his picture books means he has some amount of emotion; Sai and his brother have a bond that he cannot break. Sai is intrigued by the idea, saying that, although he and his brother were not actually related, Sai had wanted to give him the picture book to symbolise their relationship as a family. Team 7 restrains him and prepares to go looking for Sasuke. Sai tells Naruto he is wasting his time, but Naruto insists that he and Sasuke have a bond of friendship that he would do anything to protect.