Musannaf Ibn Abi Shayba 1:932

Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, from Abu Hurayrah, who said—and Yunus said: "I do not know it except that he attributed it to the Prophet (ﷺ)": "When he sits between her four limbs, then exerts himself, Ghusl (ritual bathing) becomes obligatory, whether he ejaculated or did not ejaculate."