Musnad Ahmad 1153:2212

Yunus narrated to us, he said: Hammad—meaning Ibn Salamah—narrated to us, from Hisham ibn Urwah, from his father, from Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you experiences sleepiness while he is praying, let him sleep until his sleepiness departs. For perhaps one of you might go to seek forgiveness from Allah, but instead he insults himself."