Musnad Ahmad 32:406

'Abd al-Rahman ibn Mahdi narrated to us, Sufyan narrated to us, from 'Amr ibn 'Amir, who said: I heard Anas saying: "The Messenger of Allah, peace and blessings of Allah be upon him, used to perform ablution for every prayer." He said: I said: "And you, how did you use to do?" He said: "We used to pray the prayers with one ablution, as long as we did not break it."