Musnad Ahmad 30:1453

'Affan narrated to us, Hammam and Aban narrated to us, they said: Qatadah informed us, 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 and exerts himself, then Ghusl (ritual bathing) has become obligatory, whether he ejaculated or did not ejaculate."