Musnad Ahmad 32:578

'Abd al-Samad narrated to us, my father narrated to us, Ayyub narrated to us, from Abu Qilabah, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you becomes drowsy while he is in prayer, let him depart and sleep until he knows what he is saying."