Yahya related to me from Malik from Yahya ibn Said from Said ibnal-Musayyab that Abu Musa al-Ashari came to A'isha, the wife of theProphet, may Allah bless him and grant him peace, and said to her,"The disagreement of the companions in a matter which I hate to bringbefore you has distressed me." She said, "What is that? You did notask your mother about it, so ask me." He said, "A man penetrates hiswife, but becomes listless and does not ejaculate. "She said, "Whenthe circumcised part passes the circumcised part ghusl is obligatory."Abu Musa added, "I shall never ask anyone about this after you."
USC-MSA web (English) reference: Book 2, Hadith 75
Arabic reference: Book 2, Hadith 105