Musnad Ahmad 1153:1263

Al-Walid ibn Muslim narrated to us, Al-Awza'i narrated to us, he said: Abd al-Rahman ibn al-Qasim narrated to me, from his father, from Aisha, the wife of the Prophet, may God bless him and grant him peace, she said: "When the circumcised part passes the circumcised part, then Ghusl (ritual bathing) has become obligatory. I and the Messenger of God, may God bless him and grant him peace, did it, and we performed Ghusl."