Muhammad ibn 'Awf al-Ta'i narrated to us: Ahmad ibn Khalid narrated to us: Muhammad ibn Ishaq narrated to us, from Muhammad ibn Yahya ibn Habban, from 'Abdullah ibn 'Abdullah ibn 'Umar, who said: I asked, "What is your view regarding Ibn 'Umar's performing ablution for every prayer, whether he is in a state of purity or not? What is the reason for that?" He replied: Asma', daughter of Zayd ibn al-Khattab, narrated to me that 'Abdullah ibn Hanzalah ibn Abi 'Amir narrated to her that the Messenger of Allah (may Allah bless him and grant him peace) was commanded to perform ablution for every prayer, whether he was pure or not. When that became burdensome for him, he was commanded to use the siwak for every prayer. Ibn 'Umar felt that he possessed the strength for it, so he did not stop performing ablution for every prayer.
Abu Dawud said: Ibrahim ibn Sa'd narrated it from Muhammad ibn Ishaq, and he said: 'Ubaydullah ibn 'Abdullah.