Musnad Ahmad 27:679

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, I heard Abu Ishaq—and Hajjaj said in his narration: from Abu Ishaq—I heard Yahya ibn Waththab, that he asked Ibn Umar about the ghusl (ritual bath) on the day of Jumu'ah (Friday). He said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded us to do it."