Musnad Ahmad 3:60

Hassan ibn Musa narrated to us, Shaiban narrated to us, from Yahya ibn Abi Kathir, from Abu Salamah, that Ata ibn Yasar informed him, from Zayd ibn Khalid al-Juhani, that he informed him: That he asked Uthman ibn Affan, he said: I said: "What is your opinion if a man has intercourse with his wife and does not ejaculate?" Uthman said: "He performs ablution as he does for prayer, and he washes his private part." He said: And Uthman said: "I heard it from the Messenger of Allah, peace and blessings of Allah be upon him." So I asked about that Ali ibn Abi Talib, and al-Zubayr, and Talhah, and Ubayy ibn Ka'b, and they commanded him to do that.