Musnad Ahmad 918:2

'Ali ibn Ishaq narrated to us, 'Abdullah—meaning Ibn al-Mubarak—informed us, Yunus informed me, from al-Zuhri, from Sahl ibn Sa'd al-Ansari—and he had reached the Prophet, may Allah bless him and grant him peace, while he was fifteen years old—he said: Ubayy ibn Ka'b narrated to me: That the legal opinion (fatwa) which they used to give regarding their saying, "Water is from water," was a concession that was granted at the beginning of Islam, then we were commanded to perform ghusl (full ritual washing) after it.