Musnad Ahmad 27:693

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Abu Ishaq, [who said]: I heard Yahya ibn Waththab narrating from Ibn 'Umar, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "Whoever comes to the Friday [prayer], let him perform ghusl (a ritual bath)."