Muhammad ibn Yahya ibn Habban asked Abdullah ibn Abdullah ibn Umar about the reason for Ibn Umar's performing ablution for every prayer, whether he was with or without ablution. He replied: Asma', daughter of Zayd ibn al-Khattab, reported to me that Abdullah ibn Hanzalah ibn AbuAmir narrated to her that the Messenger of Allah ﷺ was earlier commanded to perform ablution for every prayer whether or not he was with ablution. When it became a burden for him, he was ordered to use tooth-stick for every prayer. As Ibn Umar thought that he had the strength (to perform the ablution for every prayer), he did not give up performing ablution for every prayer. Abu Dawud said: Ibrahim b. Sa'd narrated this tradition on the authority of Muhammad b. Ishaq, and there he mentions the name of 'Ubaid Allah b. 'Abd Allah (instead of 'Abd Allah b. 'Abd Allah b. 'Umar)  

أَنَّ رَسُولَ اللَّهِ ﷺ أُمِرَ بِالْوُضُوءِ لِكُلِّ صَلاَةٍ طَاهِرًا وَغَيْرَ طَاهِرٍ فَلَمَّا شَقَّ ذَلِكَ عَلَيْهِ أُمِرَ بِالسِّوَاكِ لِكُلِّ صَلاَةٍ فَكَانَ ابْنُ عُمَرَ يَرَى أَنَّ بِهِ قُوَّةً فَكَانَ لاَ يَدَعُ الْوُضُوءَ لِكُلِّ صَلاَةٍ  

[Machine] "The Messenger of Allah ﷺ was commanded to perform ablution before every prayer, whether he was in a state of ritual purity or not. When this became burdensome for the Messenger of Allah ﷺ, he was then commanded to use a toothbrush (siwak) before every prayer, and ablution was no longer required for him except in cases of ritual impurity."  

أَنَّ رَسُولَ اللهِ ﷺ كَانَ أُمِرَ بِالْوُضُوءِ لِكُلِّ صَلَاةٍ طَاهِرًا كَانَ أَوْ غَيْرَ طَاهِرٍ فَلَمَّا شَقَّ ذَلِكَ عَلَى رَسُولِ اللهِ ﷺ أُمِرَ بِالسِّوَاكِ عِنْدَ كُلِّ صَلَاةٍ وَوُضِعَ عَنْهُ الْوُضُوءُ إِلَّا مِنْ حَدَثٍ