Musnad Ahmad 1153:800

Abu Nu'aym narrated to us, he said: Sufyan narrated to us, from Ali ibn Zayd, from Sa'id ibn al-Musayyib, from Aisha, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When he sits between the four limbs, and then touches the circumcised part with the circumcised part, then Ghusl (ritual bathing) has become obligatory."