Sahih Muslim Book 3, Hadith Number 597.

Chapter : It is permissible for a person to sleep after sexual intercourse (without a bath) and the desirability of ablution for him, and washing of the sexual organ, as he intends to eat, drink, or sleep or cohabit.

‘Aisha reported: Whenever the Messenger of Allah (may peace be upon him) intended to sleep after having sexual intercourse, he performed ablution as for the prayer before going to sleep.