Musnad Ahmad 1153:2007

Yazid narrated to us, he said: Hammad ibn Salamah informed us, from Thabit al-Bunani, from Abdullah ibn Rabah, from Abd al-Aziz ibn al-Nu'man, from Aisha, from the Prophet, may God bless him and grant him peace, who said: "When the two circumcised parts meet, ghusl (ritual bathing) becomes obligatory."