Sayyiduna Abu Hurayrah (may Allah be pleased with him) relates that the Prophet (peace and blessings be upon him) said: "When a man sits between her four limbs and then exerts himself, ghusl becomes obligatory." And in the hadith of Matar: "Even if there is no ejaculation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 764