Musnad Ahmad 30:3612

Sulayman ibn Dawud narrated to us, Hisham and Shu'bah narrated to us, from Qatadah, from al-Hasan, from Abu Rafi', from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When he sits between her four limbs," Shu'bah said: "then exerts himself," and Hisham said: "then strives, then Ghusl (ritual bathing) has become obligatory."