Abu Dawud Book 1, Hadith Number 48.
Chapter : The toothstick.
Narated By Abdullah ibn Hanzalah ibn AbuAmir : 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 Apostle of Allah (PBUH) 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.