Maliks Muwatta Book 2, Hadith Number 74.
Section : Obligation to Do Ghusl when the Two Circumcised Parts Meet.
Yahya related to me from Malik from Abu’n Nadr, the mawla of Umar ibn Abdullah that Abu Salamaibn Abdar-Rahman ibn Awf related that he had asked A’isha, the wife of the Prophet, may Allah bless him and grant him peace, what made ghusl obligatory. She said, “Do you know what you are like, Abu Salama? You are like a chick when it hears the cocks crowing and so crows with them. When the circumcised part passes the circumcised part, ghusl is obligatory.”