Musnad Ahmad 31:51

Sufyan narrated to us, from ‘Asim, from Abu al-Mutawakkil, from Abu Sa’id, from the Prophet, peace and blessings of Allah be upon him, who said: "He shall perform ablution when he has intercourse, and when he wishes to return [to it]." Sufyan said: Abu Sa’id witnessed the [Battle of] al-Harrah.