Musannaf Ibn Abi Shayba 1:956

Abu Mu'awiyah narrated to us, from Hajjaj, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When the two circumcised parts meet and the glans disappears, then Ghusl (ritual bathing) has become obligatory."