Musannaf Ibn Abi Shayba 1:931

Al-Fadl ibn Dukayn narrated to us, from Hisham al-Dastawa'i, from Qatadah, from al-Hasan, from Abu Rafi', from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said: "When he sits between her four limbs, then exerts himself, the ritual bath (ghusl) has become obligatory."