Musnad Ahmad 32:1074

Hajjaj narrated to us, Shu'bah narrated to us, from 'Amr ibn 'Amir al-Ansari, who said: I heard Anas ibn Malik saying: "A vessel of water was brought to the Prophet, may Allah bless him and grant him peace, and he performed ablution." He said: So I said to Anas, "Did the Prophet, may Allah bless him and grant him peace, perform ablution for every prayer?" He said, "Yes." He said: I said, "And you?" He said, "We used to pray the prayers with one ablution." He said: Then I asked him after that, and he said, "As long as we did not break our state of purity."