Sahih Muslim Book 003, Hadith Number 0600.
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 said: Is one amongst us permitted to sleep in a state of impurity (i.e. after having sexual intercourse)? He (the Holy Prophet) said: Yes, after performing ablution.