Musnad Ahmad 913:4

Abdullah narrated to us, he said: Ubaydullah ibn Umar al-Qawariri narrated to me, he said: Hammad ibn Zayd narrated to us, from Hisham ibn Urwah, from his father, he said: It reached me from Abu Ayyub ibn Zayd a hadith while he was in the land of the Byzantines. He said: So I met Abu Ayyub, and he narrated to me from Ubayy ibn Ka'b, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When a man has intercourse with his wife, then becomes lazy (i.e., does not ejaculate), he should wash what touched the woman from him, then he should perform wudu."