Musnad Abu Ya'la al-Mawsili 42:191

Narrated to us Zuhayr, narrated to us Jarir, from ‘Asim al-Ahwal, from Abu al-Mutawakkil, from Abu Sa‘id al-Khudri, from the Prophet (may Allah bless him and grant him peace), regarding the one who has intercourse then wishes to repeat it: "Let him perform ablution (wudu)."