Musnad Ahmad 32:851

Mu'ammal and 'Affan narrated to us, they said: Hammad ibn Salamah narrated to us, from Thabit, from Anas, who said: The prayer time arrived, so the neighbors of the mosque stood up to go to their homes to perform ablution. There remained in the mosque a group of the Emigrants (Muhajirun), numbering between seventy and eighty. The Messenger of Allah, peace and blessings of Allah be upon him, called for water. A stone vessel containing water was brought to him. He placed the fingers of his right hand into the vessel, and he began to pour [water] upon them while they performed ablution, and he said: "Perform ablution, come to ablution," until they had all performed ablution, and there remained in it [the vessel] about the same amount that had been in it.