Musnad Ahmad 27:1727

Ya'qub narrated to us, my father narrated to us, from Muhammad ibn Ishaq, Nafi', the freed slave of Abdullah ibn Umar, narrated to me, from Abdullah ibn Umar, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "If one of you becomes drowsy while in his sitting place on the day of Friday, let him move from it to another."