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 then exerts himself, ghusl becomes obligatory, even if there is no ejaculation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 765