Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain of narration, it is narrated from (Sayyidah Aisha, may Allah be pleased with her) that the Messenger of Allah, peace and blessings be upon him, said: "When any one of you becomes drowsy during prayer, he should sleep until the effect of sleep leaves him, because if a person prays while drowsy, it is possible that instead of seeking forgiveness, he may start cursing himself."