Musnad Ahmad 28:194

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