It is narrated from Muhammad bin Yahya bin Hibban that he asked Abdullah (may Allah have mercy on him), the son of Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): Tell me about Ibn Umar (may Allah be pleased with him) performing ablution for every prayer, whether he was in a state of ablution or not; why did he do that? He said: Asma bint Zayd bin al-Khattab (may Allah be pleased with her) narrated to me that Abdullah bin Hanzalah bin Abi Amir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) was ordered to perform ablution for every prayer, whether he was in a state of ablution or not. When this became difficult for him (peace and blessings be upon him), he was ordered to use the siwak for every prayer. Ibn Umar (may Allah be pleased with him) held that he had the strength, so he would not leave performing ablution for every prayer.
(b) Sa'id said that in the narration of Ubaydullah bin Abdullah the words are: "When this became difficult for them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 158