Imam Malik's Muwatta, Chapter: Purity

Yahya related to me from Malik from Yahya ibn Said from Said ibn al-Musayyab that Abu Musa al-Ashari came to A'isha, the wife of the Prophet, may Allah bless him and grant him peace, and said to her, "The disagreement of the companions in a matter which I hate to bring before you has distressed me." She said, "What is that? You did not ask your mother about it, so ask me." He said, "A man penetrates his wife, but becomes listless and does not ejaculate. "She said, "When the circumcised part passes the circumcised part ghusl is obligatory." Abu Musa added, "I shall never ask anyone about this after you."