Sayyiduna Abu Hurairah (may Allah be pleased with him) narrated from the Messenger of Allah (peace and blessings be upon him), he (peace be upon him) said: "When he sits between her four limbs and then exerts himself (has intercourse), then ghusl (ritual bath) becomes obligatory upon him."
Hadith Referenceمسند اسحاق بن راهويه / كتاب الطهارة / 82