Musnad Abu Ya'la al-Mawsili 113:10

Ja'far ibn Mihran al-Sabbak narrated to us, 'Abd al-Warith narrated to us, from Ayyub, from Abu Qilabah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you becomes drowsy while he is praying, let him depart [from the prayer] until he understands what he is saying."