Musnad Ahmad 27:559

'Umar ibn 'Ubayd al-Tanafisi narrated to us, from Abu Ishaq—meaning al-Sabi'i—from Nafi', from Ibn 'Umar, who said: I heard the Prophet, may Allah bless him and grant him peace, upon the pulpit saying: "Whoever comes to the Friday [prayer], let him perform ghusl (a ritual bath)."