Musannaf Ibn Abi Shayba 1:929

Abu Bakr narrated to us, he said: Isma'il bin 'Ulayyah narrated to us, from 'Ali bin Zayd bin Jud'an, from Sa'id bin al-Musayyib, from 'Aishah, she said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When he sits between the four limbs, and the circumcision [part] touches the circumcision [part], then Ghusl (ritual bathing) has become obligatory."