In Chapter 3 of 'Harry Potter and the Prisoner of Azkaban' titled 'The Knight Bus', Harry finds out that Sirius Black is 'after' him. He overhears a conversation between Cornelius Fudge and Cornelius' friend, where they discuss Sirius' escape from Azkaban and his potential threat to Harry.
In the third chapter of Harry Potter and the Prisoner of Azkaban, titled 'The Knight Bus', Harry finds out that Sirius Black is 'after' him.
Harry overhears Cornelius Fudge, the Minister of Magic, and Cornelius' friend, discussing Sirius Black's escape from Azkaban prison and his supposed connection to Harry.
They mention that Sirius has been sighted near Hogwarts and may be coming after Harry. This information leaves Harry feeling unsettled and worried.
At the end of the chapter, Harry has a conversation with Hermione, where she confirms the rumors about Sirius Black and suggests that he might be trying to fulfill Voldemort's unfinished task - killing Harry Potter.