The Messenger of Allah ﷺ commanded that ablution be performed before every prayer, whether one was in a state of purity or not. When this became difficult for him, he was commanded to use the toothstick before every prayer instead. Ibn Umar believed he had the strength for it, so he never abandoned performing ablution for every prayer.
أن رسول الله ﷺ أمر بالوضوء لكل صلاة طاهرا وغير طاهر، فلما شق ذلك عليه أمر بالسواك لكل صلاة. فكان ابن عمر يرى أن به قوة؛ فكان لا يدع الوضوء لكل صلاة.
Abu Dawud (48) narrated it, saying, Muhammad bin Awf Tai told us, Ahmad bin Khalid told us, Muhammad bin Ishaq told us, from Muhammad bin Yahya bin Habban, from Abdullah bin Abdullah bin Umar, who said, I said, "What do you think about Ibn Umar's performing wudu for every prayer, whether he was in a state of purity or not, why is that?" He said, "Asma bint Zayd bin Khattab told me that Abdullah bin Hanzala told her," and he related the hadith.Its isnad is hasan, though Muhammad bin Ishaq is a mudallis narrator. However, an explicit statement of transmission appears in Imam Ahmad (21960) from Yaqub bin Ibrahim, who said, my father told us, from Muhammad bin Ishaq, Muhammad bin Yahya bin Habban Ansari, then Mazini, from Mazin of Banu Najjar, told me, from Ubaydullah bin Abdullah bin Umar, and he related the hadith, except that he said in it Ubaydullah bin Umar, with the diminutive form.Abu Dawud pointed to the narration of Ibrahim, that is, Ibn Saad, from Muhammad bin Ishaq, in which the name appears as Ubaydullah, in the diminutive form.I say, this discrepancy does no harm, for both Abdullah, in the non-diminutive form, and Ubaydullah, in the diminutive form, are trustworthy narrators among the men of the two Shaykhs, and Abu Zura and Nasai both affirmed their trustworthiness.