Yahya related to me from Malik from Hisham ibn Urwa from hisfather from A'isha, the wife of the Prophet, may Allah bless him andgrant him peace, that the Messenger of Allah, may Allah bless him andgrant him peace, said, "If you are drowsy in prayer, sleep until sleepleaves you, because if you pray while you are drowsy, you do not knowwhether you may intend to ask for forgiveness but (in fact) ask forharm."
App reference: Book 7, Hadith 3
USC-MSA web (English) reference: Book 7, Hadith 3
Arabic reference: Book 7, Hadith 257