Setup
1Harry Potter and the Philosopher's Stone · The scar and the shared enemy
The scar hurts when Voldemort is near. The first book treats that as the mark of the boy who lived. It does not yet say that something of Voldemort is living in Harry.
2Harry Potter and the Chamber of Secrets · Parseltongue
Harry speaks to snakes and understands the basilisk. He fears he belongs in Slytherin. Dumbledore tells him he has gifts from Voldemort, and that choosing matters more than the gift. The sentence is the clue, spoken before anyone says “Horcrux.”
3Harry Potter and the Order of the Phoenix · The visions
Harry occupies Voldemort’s mind often enough that a vision of Sirius is used to trap him. Dumbledore starts lessons about the connection and then stops short of the last conclusion.
Payoff
Harry Potter and the Deathly Hallows · Snape’s memories, then the Forest
Snape’s memories show Dumbledore explaining that Harry himself is the Horcrux Voldemort never meant to make. Voldemort has to strike him, and Harry has to walk out to that strike, before the piece of soul in the scar can die and the man can be killed.
