"'You can speak Parseltongue, Harry,' said Dumbledore calmly, 'because Lord Voldemort--who is the last remaining ancestor of Salazar Slytherin--can speak Parseltongue. Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure...'"
Chamber of Secrets pg 245
"'... Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort's soul was blasted apart from the whole, and latched itself on to the only living soul left in that collapsing building. Part of Lord Voldemort lives inside Harry, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort's mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to, and protected by Harry, Lord Voldemort cannot die.'"
Deathly Hallows pg 551
"'You were the seventh Horcrux, Harry, the Horcrux he never meant to make. He had rendered his soul so unstable that it broke apart when he committed those acts of unspeakable evil, the murder of your parents, the attempted killing of a child. But what escaped from that room was even less than he knew. he left more than his body behind. He left part of himself latched to you, the would-be victim who had survived."
Deathly Hallows page 568
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