The Messenger of Allah ﷺ said, "When a man sits between the four parts of a woman and then exerts himself with her, the ritual bath becomes obligatory."
حدثنا أبو بكر بن أبي شيبة حدثنا الفضل بن دكين عن هشام الدستوائي عن قتادة عن الحسن عن أبي رافع
عن رسول الله ﷺ قال إذا جلس الرجل بين شعبها الأربع ثم جهدها فقد وجب الغسل
Bukhari included it in the Book of Ghusl, chapter "When the two circumcised parts meet," no. 291, and Muslim in the Book of Menstruation, chapter "The abrogation of 'water is required only for water' and the obligation of ghusl when the two circumcised parts meet," no. 348, from the hadith of Hisham, as narrated by him. The hadith has many corroborating narrations.