Musannaf Ibn Abi Shayba 1:953

Ibn ‘Ulayyah narrated to us, from Abu ‘Awn, from ‘Abd al-Rahman ibn Abi Layla, that he heard it from ‘Umar—or from his brother, that he heard it from ‘Umar—he said: "When the circumcised part passes the circumcised part, then Ghusl (ritual bathing) has become obligatory."