Musnad Ahmad 918:5

Abu al-Yaman narrated to us, Shu'ayb informed us, from al-Zuhri, who said: Sahl ibn Sa'd al-Ansari—and he had seen the Prophet, may God bless him and grant him peace, and heard from him, and he mentioned that he was fifteen years old when the Prophet, may God bless him and grant him peace, passed away—said: Ubayy ibn Ka'b narrated to me: That the legal opinion (fatwa) which they used to give was a concession that the Prophet, may God bless him and grant him peace, had permitted in the beginning of Islam, then he commanded performing ghusl (ritual bathing) after it.