Yahya related to me from Malik from Hisham ibn Urwa from hisfather that A'isha, the wife of the Prophet, may Allah bless him andgrant him peace, used to say, "If you have intercourse with your wifeand then wish to go to sleep before doing ghusl, do not sleep untilyou have done wudu as for prayer."
USC-MSA web (English) reference: Book 2, Hadith 79
Arabic reference: Book 2, Hadith 109