(a) It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When a man sits between her four limbs and the private parts meet, ghusl becomes obligatory."
(b) In the hadith of Wahb bin Jarir: "When a man sits between her four limbs and then exerts himself, ghusl becomes obligatory." And in the hadith of Abu Nu'aym there are also these words: "then exerts himself."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 763