Harry Potter and the Sorcerer's Stone
How we cite our quotes:
Your mother died to save you. If there is one thing Voldemort cannot understand, it is love. He didn't realize that love as powerful as your mother's for you leaves its own mark. Not a scar, no visible sign… to have been loved so deeply, even though the person who loved us is gone, will give us some protection forever. (17.120)
Even though Harry's parents are gone, in a way they're still with him, "mark[ed]" on his very skin. In a way, there's no greater measure of love than giving your life for another person, and that's what Harry's mother did for him. That kind of sacrifice forms a magic spell more powerful than any other in this novel.