Musnad Abu Ya'la al-Mawsili 156:4

Shayban narrated to us, Jarir ibn Hazim narrated to us, al-Hasan narrated to us, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When he sits between her four limbs, then exerts himself, then the ritual bath (ghusl) has become obligatory upon both of them, even if he does not emit (semen)."