Ashwatthama assassinated members of the Pandava camp in the dead of the night when they were sleeping. It is a heinous crime.
One should not cast weapons against kine, brahmanas, kings, women, friends, one's own mother, one's own preceptor, a weak man, an idiot, a blind man, a sleeping man, a terrified man, one just got up from sleep, an intoxicated person, a lunatic and one that is careless. The ancient preceptors always preached this truth to men. - Mahabharata X.6.21
What punishment did he get for this crime? Please refer to the Verse numbers from Mahabharata.
Clarification - I know that Ashwatthama was cursed by Krishna for living a wretched life for 3000 years. But that was due to his sin of directing his Brahmastra to an unborn foetus. Here I am asking about his punishment for killing sleeping men.