Musnad Ahmad 27:1378

Abu al-Mughirah narrated to us, al-Awza'i narrated to us, Yahya narrated to me, from Nafi', from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you comes to the Friday (prayer), let him perform ghusl (ritual bath)."