Musnad Abu Ya'la al-Mawsili 42:154

Ishaq narrated to us, Sufyan narrated to us, from Safwan bin Sulaym, from Ata bin 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."