Musnad Abu Ya'la al-Mawsili 113:13

Ya'qub ibn Ibrahim al-Dawraqi narrated to us, Muhammad ibn 'Abd al-Rahman al-Tafawi narrated to us, from Ayyub, from Abu Qilabah, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "If one of you becomes drowsy while in prayer, let him depart and sleep."