Yahya related to me from Malik from Abu'n Nadr, the mawla of Umaribn Abdullah that Abu Salamaibn Abdar-Rahman ibn Awf related that hehad asked A'isha, the wife of the Prophet, may Allah bless him andgrant him peace, what made ghusl obligatory. She said, "Do you knowwhat you are like, Abu Salama? You are like a chick when it hears thecocks crowing and so crows with them. When the circumcised part passesthe circumcised part, ghusl is obligatory." (DE)
USC-MSA web (English) reference: Book 2, Hadith 74 (DE)
Arabic reference (DE): Book 2, Hadith 104 (DE)