“The Dark Lord will return, and Harry Potter will be in terrible danger when he does.”

There was a long pause, and slowly Snape regained control of himself, mastered his own breathing. At long last he said, “Very well. Very well. But never — never tell, Dumbledore! This must be between us! Swear it! I cannot bear . . . especially Potter’s son . . . I want your word!”

“My word, Severus, that I shall never reveal the best of you?” Dumbledore sighed, looking down into Snape’s ferocious, anguished face. “If you insist . . .”

Harry Potter and the Deathly Hallows – J.K. Rowling