Musnad Ahmad 32:424

'Abd al-Rahman ibn Mahdi narrated to us, Sufyan narrated to us, from 'Amr ibn 'Amir, who said: I heard Anas ibn Malik say: "The Messenger of Allah (peace and blessings of Allah be upon him) used to perform ablution for every prayer." I said: "And you, how did you used to do?" He said: "We used to pray [multiple] prayers with one ablution."