Mustadrak al-Hakim — Hadith 563

Muhammad bin Yahya bin Habban narrates from Ubaydullah bin Abdullah bin Umar; he said: I said to him: "Tell me about the ablution of Abdullah bin Umar for every prayer, whether he was in a state of ablution or not: from whom did he take it?" He said: Asma bint Zayd bin al-Khattab told him (Ibn Umar) that Sayyiduna Abdullah bin Hanzalah al-Ghasil (may Allah be pleased with him) told her: "The Messenger of Allah (peace and blessings be upon him) was commanded to perform ablution at the time of every prayer, whether he was in a state of ablution or not. Then, when this became difficult for the Messenger of Allah (peace and blessings be upon him), he was commanded to use the tooth-stick (siwak) at the time of every prayer, and the obligation of ablution (for one who already has it) was lifted, except when ablution is broken." And Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) felt he had the strength for it, so he kept doing so until he died.
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it. They agreed only on (the narration) that he (peace and blessings be upon him) used to perform ablution for every prayer, but in the year of the Conquest of Makkah he prayed all the prayers with a single ablution.