Musnad Ahmad 27:1033

Muhammad—meaning Ibn Ja'far—narrated to us, Shu'bah narrated to us, from Al-Hakam, from Nafi', from Ibn 'Umar, who said: The Prophet, may Allah bless him and grant him peace, delivered a sermon and said: "When one of you goes to the Friday prayer, let him perform ghusl (a ritual bath)."