Musnad Ahmad 1153:638

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from ‘Ali ibn Zayd, from Sa‘id ibn al-Musayyib, that Abu Musa said to ‘Aishah: "I want to ask you about something, and I am shy before you." She said: "Ask, and do not be shy, for I am only your mother." So he asked her about a man who has intercourse but does not ejaculate. She said, regarding the Prophet, peace and blessings of Allah be upon him: "When the circumcised part touches the circumcised part, then Ghusl (ritual bathing) has become obligatory."