Musnad Ahmad 27:1560

Ishaq ibn Isa narrated to us, Laith narrated to me, Ibn Shihab and Yunus narrated to me, they said: Laith narrated to us, from Ibn Shihab, from Abdullah ibn Abdullah, from Abdullah ibn Umar, from the Messenger of Allah (peace and blessings of Allah be upon him) that he said while he was on the pulpit: "Whoever among you comes to the Friday [prayer], let him perform ghusl (ritual bath)."