I asked, "What about Ibn Umar's performing ablution for every prayer, whether in a state of purity or not? What is the basis for that?"He replied, "Asma bint Zayd bin al-Khattab told me that Abdullah bin Hanzala bin Abu Amir reported to her that the Messenger of Allah ﷺ commanded ablution before every prayer, whether one was in a state of purity or not. When this became burdensome for him, he was instructed to use the tooth-stick before every prayer instead. Ibn Umar, however, considered himself strong enough to manage it, so he never abandoned ablution for every prayer."
حدثنا محمد بن عوف الطائي حدثنا أحمد بن خالد حدثنا محمد بن إسحق عن محمد بن يحيى بن حبان
قلت أرأيت توضؤ ابن عمر لكل صلاة طاهرا وغير طاهر عم ذاك فقال حدثتنيه أسماء بنت زيد بن الخطاب أن عبد الله بن حنظلة بن أبي عامر حدثها 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
[Its isnad is hasan] Ahmad reported it in 5/225 from the hadith of Muhammad bin Ishaq through this route, and Ibn Khuzayma graded it sahih, no. 15, as did Hakim according to the standard of Muslim in 1/156, and Dhahabi agreed with him. Ibn Ishaq explicitly stated that he heard it directly.