Musnad Ahmad 32:506

'Abd al-Samad narrated to us, my father narrated to me, 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 praying, let him depart and sleep, until he knows what he is saying."