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 ﷺ 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."
أَنَّهُ قَالَ سَأَلْتُ عَائِشَةَ زَوْجَ النَّبِيِّ ﷺ 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