Musnad Ahmad 30:1986

Abu Nu'aym narrated to us, Hisham narrated to us, from Qatadah, from al-Hasan, from Abu Rafi', from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "When he sits between her four limbs, then exerts himself, the bath (ghusl) has become obligatory."