Musnad Ahmad 1153:1580

Waki’ narrated to us, he said: Salih bin Abi al-Akhdar narrated to us, from al-Zuhri, from ‘Urwah and Abu Salamah, from ‘Aishah, that the Prophet, peace and blessings of Allah be upon him, "when he wanted to eat while he was in a state of janabah (major ritual impurity), he would wash his hands."