Musnad Abu Ya'la al-Mawsili 150:422

Muslim al-Jarmi narrated to us, Makhlad narrated to us, from Hisham, from Ibn Sirin, from Abu Hurayrah, who said: They were discussing in a circle I was in what necessitates Ghusl (ritual bathing). Some of them said: "When the man has intercourse." Others said: "Until the fluid (semen) is emitted." I said: "I will bring you the knowledge of that." So I went to Aisha, the Mother of the Believers, and said to her: "O Mother of the Believers, I want to ask you about something, but I am shy to ask you about it." She said: "Do not be shy to ask me about what you would ask your mother about, for I am indeed your mother." I said: "O Mother, what necessitates Ghusl?" She said: "You have come to one who is well-informed. When he sits between her four limbs and the two circumcised parts meet, then Ghusl has become necessary."