Abu Dawud Book 1, Hadith Number 62.

Chapter : A man may renew the ablution without defilement.

Narated By Abdullah ibn Umar : Abu Ghutayf al-Hudhali reported: I was in the company of Ibn Umar. When the call was made for the noon (zuhr) prayer, he performed ablution and said the prayer. When the call for the afternoon (‘asr) prayer was made, he again performed ablution. Thus I asked him (about the reason of performing ablution). He replied: The Apostle of Allah (PBUH) said: For a man who performs ablution in a state of purity, ten virtuous deeds will be recorded (in his favour).

Abu Dawud said: This is the tradition narrated by Musaddad, and it is more perfect.