Musnad Ahmad 956:4

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn Yahya ibn Habban al-Ansari—then al-Mazini, of the Mazin of Banu al-Najjar—narrated to me, from Ubayd Allah ibn Abd Allah ibn Umar, who said: I said to him, "Have you seen the ablution of Abd Allah ibn Umar for every prayer, whether he was pure or impure; what is it based on?" He said: Asma bint Zayd ibn al-Khattab narrated to him that Abd Allah ibn Hanzala ibn Abi Amir al-Ghasil narrated to her: "That the Messenger of Allah, peace and blessings of Allah be upon him, had commanded ablution for every prayer, whether one was pure or impure. When that became difficult for the Messenger of Allah, peace and blessings of Allah be upon him, he commanded the use of the siwak at every prayer, and he lifted the requirement of ablution from him, except for one who has had a hadath (a break in ritual purity)." He said: "Abd Allah used to believe that he had the strength for that, so he used to do it until he died."