Musnad Ahmad 736:1

Abu Ahmad al-Zubayri narrated to us, Kathir ibn Zayd narrated to us, from al-Muttalib ibn Abdullah, from Itban, or the son of Itban, the Ansari, who said: I said: "O Prophet of Allah, I was with my wife, and when I heard your voice, I desisted and performed ghusl (ritual washing)." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Water is from water."