Musannaf Ibn Abi Shayba 3:2644

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from al-Hakam, from Nafi', from Ibn 'Umar: That the Messenger of Allah (ﷺ) delivered a sermon on the day of Friday, and he said: "Whoever goes to the Friday (prayer), then let him perform ghusl."