It was because of your mother. She sacrificed herself for you. And that kind of act leaves a mark.
10 seconds sound clip from the Harry Potter and the Sorcerer's Stone movie soundboard.
You can hear this line at 02:22:56 in the Blu-ray version of the movie.
Quote context
[...]
- That is one of my more brilliant ideas.
- And between you and me, that is saying something.
- Does that mean, with the Stone gone that is, that Voldemort can never come back?
- I'm afraid... there are ways in which he can return.
- Harry, do you know why Professor Quirrell couldn't bear to have you touch him?
- It was because of your mother. She sacrificed herself for you. And that kind of act leaves a mark.
- No, this kind of mark cannot be seen. It lives in your very skin.
- Bertie Bott's Every Flavor Beans.
- But I think I could be safe with a nice toffee.
[...]