Sahih Muslim Book 3, Hadith Number 602.

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.

Ibn Umar reported: Umar b. al-Khattab said to the Messenger of Allah (may peace be upon him), that he became Junbi during the night. The Messenger of Allah (may peace be upon him) said to him: Perform ablution, wash your sexual organ and then go to sleep.