The Prophet ﷺ said, "When a man sits between her four limbs and then exerts himself, the ritual bath becomes obligatory."
حدثنا أبو نعيم حدثنا هشام عن قتادة عن الحسن عن أبي رافع
عن النبي ﷺ قال إذا جلس بين شعبها الأربع ثم جهد فقد وجب الغسل
Its isnad is sahih. It was previously mentioned at 7197. The hadith was also narrated by Nasai, 1/110, no. 191, in the Book of Purification, and Ibn Majah, 1/200, no. 610, in the Book of Purification, chapter on what has been said regarding the obligation of ghusl when the two circumcised parts meet.