Muwatta Malik, Book 2, Hadith 74 (USC-MSA) ⓘ

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي النَّضْرِ، مَوْلَى عُمَرَ بْنِ عُبَيْدِ اللَّهِ عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ، أَنَّهُ قَالَ سَأَلْتُ عَائِشَةَ زَوْجَ النَّبِيِّ صلى الله عليه وسلم مَا يُوجِبُ الْغُسْلَ فَقَالَتْ هَلْ تَدْرِي مَا مَثَلُكَ يَا أَبَا سَلَمَةَ مَثَلُ الْفَرُّوجِ يَسْمَعُ الدِّيَكَةَ تَصْرُخُ فَيَصْرُخُ مَعَهَا إِذَا جَاوَزَ الْخِتَانُ الْخِتَانَ فَقَدْ وَجَبَ الْغُسْلُ ‏.‏