Musnad Ahmad 1153:1643

Yahya narrated to us, Hisham and Waki’ narrated to us, from Hisham—the meaning is the same—he said: My father informed me, from Aisha, from the Prophet, peace and blessings of Allah be upon him, who said: "If one of you becomes drowsy while he is praying, let him sleep until the sleep leaves him. For if he prays while he is drowsy, perhaps he might go to seek forgiveness and instead curse himself."