Muwatta Malik #258

Yahya related to me from Malik from Hisham ibn Urwa from his father from A'isha, the wife of the Prophet, may Allah bless him and grant him peace, that the Messenger of Allah, may Allah bless him and grant him peace, said, "If you are drowsy in prayer, sleep until sleep leaves you, because if you pray while you are drowsy, you do not know whether you may intend to ask for forgiveness but (in fact) ask for harm."