Musannaf Ibn Abi Shayba 1:948

Sahl ibn Yusuf narrated to us, from Shu'bah, from Sayf ibn Wahb, from Abu Harb ibn Abi al-Aswad al-Du'ali, from 'Amirah ibn Yathribi, from his father, who said: "When their two meeting places meet beyond the circumcision, the ritual bath (ghusl) becomes obligatory."