Musnad Ahmad 32:1790

Aswad ibn Amir and Hajjaj narrated to us, they said: Shu'bah narrated to us, 'Amr ibn 'Amir al-Ansari narrated to me, he said: I heard Anas ibn Malik saying: "The Prophet, peace and blessings of Allah be upon him, was brought a cup of water, and he performed ablution." 'Amr said: I said to Anas, "Did he perform ablution for every prayer?" He said, "Yes." I said, "And you?" He said, "We used to pray the prayers with one ablution." Then I asked him later, and he said, "As long as he did not break his ablution."