Musnad Ahmad 918:1

Uthman ibn Umar narrated to us, Yunus informed us, from al-Zuhri, who said: Sahl al-Ansari said—and he had reached the time of the Prophet, may Allah bless him and grant him peace, while he was fifteen years old in his time—Ubayy ibn Ka'b narrated to me: That the legal opinion they used to say, "Water is from water," was a concession that the Messenger of Allah, may Allah bless him and grant him peace, had granted in the beginning of Islam, then he commanded us to perform ghusl (full ritual washing) after it.