The Messenger of Allah ﷺ said, "When a man sits between her four limbs and then exerts himself, the bath becomes obligatory."
أخبرنا محمد بن عبد الأعلى قال حدثنا خالد قال حدثنا شعبة عن قتادة قال سمعت الحسن يحدث عن أبي رافع
أن رسول الله ﷺ قال إذا جلس بين شعبها الأربع ثم اجتهد فقد وجب الغسل
Graded by Bukhari, ghusl, chapter "when the two circumcised parts meet," hadith 291, and Muslim, menstruation, chapter "abrogation of 'water is required only from water' and the obligation of ghusl when the two circumcised parts meet," hadith 348, from the hadith of Qatada, transmitted through him. It is also found in Al-Kubra, hadith 197.