Musnad Ahmad 27:629

He said: I found in my father’s book: Hajjaj narrated to us, Shu’bah informed us, from Abu Ishaq, [who said]: I heard Yahya ibn Waththab [saying]: I asked Ibn Umar about the ghusl (ritual bath) on the day of Jumu’ah. He said: He replied, "The Messenger of Allah, may Allah bless him and grant him peace, commanded us to do it."