It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the circumcised part passes beyond the circumcised part, the ritual bath (ghusl) becomes obligatory."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7924