Musnad Ahmad 32:751

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Thabit and Qatadah, from Anas, who said: Some of the companions of the Messenger of Allah (peace and blessings of Allah be upon him) looked for water for ablution, but they did not find any. He said: So the Prophet (peace and blessings of Allah be upon him) said: "Is there water here?" He said: So I saw the Prophet (peace and blessings of Allah be upon him) place his hand in the vessel that contained the water, then he said: "Perform ablution in the name of Allah." I saw the water gushing from between his fingers, and the people were performing ablution, until they had all performed ablution to the last of them. Thabit said: I said to Anas: "How many do you think they were?" He said: "Around seventy."