Maliks Muwatta Book 2, Hadith Number 73.
Section : Obligation to Do Ghusl when the Two Circumcised Parts Meet.
Yahya related to me from Malik from Ibn Shihab from Said ibn al-Musayyab that Umar ibn al-Khattab and Uthman ibn Affan and A’isha, the wife of the Prophet, may Allah bless him and grant him peace, used to say, “When the circumcised part touches the circumcised part, ghusl is obligatory.”