Sayyiduna Abu Umamah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When the private part passes the private part, the ritual bath (ghusl) becomes obligatory."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; in its chain it is mentioned that Ja'far bin al-Zubayr narrated it from al-Qasim, and both of them are da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1443