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, thewife of the Prophet, may Allah bless him and grant him peace, used tosay, "When the circumcised part touches the circumcised part, ghusl isobligatory."
USC-MSA web (English) reference: Book 2, Hadith 73
Arabic reference: Book 2, Hadith 103