Musnad Ahmad 32:623

Aswad ibn Amir narrated to us, Sharik narrated to us, from Amr ibn Amir al-Ansari, from Anas ibn Malik, who said: We asked him about performing ablution for every prayer. He said: "As for the Prophet, may Allah bless him and grant him peace, he used to perform ablution for every prayer." "And as for us, we used to perform [multiple] prayers with a single purification."