Musnad Ahmad 31:42

Sufyan narrated to us, from Safwan ibn Sulaym, from ‘Ata’ ibn Yasar, from Abu Sa’id, in a narration—and he said once: He attributes it to the Prophet, may Allah bless him and grant him peace—he said: "Ghusl (full ritual washing) on the day of Jumu’ah." He said: "It is obligatory upon every person who has reached puberty."