The Prophet ﷺ said, "When a man sits between her four limbs and exerts himself, the bath becomes obligatory."
حدثنا يحيى عن أشعث عن الحسن
عن النبي ﷺ قال إذا جلس بين شعبها الأربع واجتهد فقد وجب الغسل
Its chain of transmission is sahih, and it has already been mentioned at 9082.