Sahih Muslim Book 003, Hadith Number 0605.
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.
Abu Sa'id al-Khudri reported: The Messenger of Allah (may peace be upon him) said: When anyone amongst you has sexual intercourse with his wife and then he intends to repeat it, he should perform ablution. In the Hadith transmitted by Abu Bakr. (the words are): "Between the two (acts) there should be an ablution," or he (the narrator) said: "Then he intended that it should be repeated."