Musannaf Ibn Abi Shayba 1:930

Waki’ narrated to us, from ‘Abdullah ibn Abi Ziyad, from ‘Ata’, from ‘A’ishah, who said: "When the circumcised part passes the circumcised part, then Ghusl (ritual bathing) has become obligatory. For that used to happen between me and the Prophet, may Allah bless him and grant him peace, and we would perform Ghusl."