Musnad Ahmad 30:3616

Wahb ibn Jarir narrated to us, Shu'bah narrated to us, and 'Abd al-Samad 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: "If he sits between her four branches, then exerts himself, then Ghusl (ritual bathing) has become obligatory." 'Abd al-Samad said: "Then he exerted himself [in her]."