Musnad Abu Ya'la al-Mawsili 42:5

Abu Khaythama narrated to us, Sufyan narrated to us, from Safwan ibn Sulaym, from ‘Ata’ ibn Yasar, from Abu Sa’id, attributing it to the Prophet (may Allah bless him and grant him peace): "Ghusl (full ritual washing) on the day of Jumu'ah is obligatory upon every person who has reached puberty."