Musnad Ahmad 1153:1681

Waki’ narrated to us, Sufyan 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 during his prayer, let him sleep, for perhaps he intends to ask for forgiveness, but instead insults himself."