Musnad Ahmad 31:588

I read to Abd al-Rahman: Malik [said]. My father said: And Abu Salamah—meaning al-Khuza'i—narrated it to us, [saying] Malik informed us, from Safwan ibn Sulaym, from 'Ata' ibn Yasar, from Abu Sa'id al-Khudri, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Ghusl (full ritual washing) on the day of Jumu'ah is obligatory upon every person who has reached puberty."