Sahih Muslim Book 003, Hadith Number 0603.
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.
'Abdullah b. Abu'l-Qais reported: I asked 'Aisha about the Witr (prayer) of the Messenger of Allah (may peace be upon him) and made mention of a Hadith, then I said: What did he do after having sexual intercourse? Did he take a bath before going to sleep or did he sleep before taking a bath? She said: He did all these. Sometimes he took a bath and then slept, and sometimes he performed ablution only and went to sleep. I (the narrator) said: Praise be to Allah Who has made things easy (for human beings).