Al-Sha'bi narrates that al-Harith narrated to him from Sayyiduna Ali (may Allah be pleased with him), Alqamah from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), and Masruq from Sayyidah Aishah (may Allah be pleased with her), that all of them said: "When the two circumcised parts meet, the ritual bath (ghusl) becomes obligatory." Masruq said: And Sayyidah Aishah (may Allah be pleased with her) was the most knowledgeable of them all about this.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9219