The Prophet ﷺ said, "When a man sits between her four limbs and the two circumcised parts touch, the ritual bath becomes obligatory."
حدثنا مسلم بن إبراهيم الفراهيدي حدثنا هشام وشعبة عن قتادة عن الحسن عن أبي رافع
أن النبي ﷺ قال إذا قعد بين شعبها الأربع وألزق الختان بالختان فقد وجب الغسل
Bukhari reported it in the chapter on ghusl, section "When the two private parts meet," hadith no. 291, from the hadith of Hisham. Muslim also reported it in the chapter on menstruation, section on the abrogation of "Water is required only after water," etc., hadith no. 348, from the hadith of Shubah, through the same chain.