Sunan Abi Dawud 48

Narrated Abdullah b. 'Abd Allah b. 'Umar: 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)