Musnad Ahmad 1153:278

Ibn Numayr narrated to us, Hisham narrated to us, from his father, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you becomes drowsy, let him sleep until the sleep leaves him. For if he prays while he is drowsy, perhaps he might go to seek forgiveness, but instead he insults himself."