Abu Dawud Book 1, Hadith Number 169.
Chapter : What a man should say after ablution.
Narated By ‘Uqbah b. ‘Amir : We served ourselves in the company of the Messenger of Allah (may peace be upon him). We tended our camels by turn. One day I had my turn to tend the camels, and I drove them in the afternoon. I found the Messenger of Allah (may peace be upon him) addressing the people. I heard him say: Anyone amongst you who performs ablution, and does it well, then he stands and offers two rak’ahs of prayer, concentrating on it with his heart and body, Paradise will be his lot by all means. I said: Ha-ha! How fine it is! A man in front of me said: The action (mentioned by the Prophet) earlier, O ‘Uqbah, is finer than this one. I looked at him and found him to be ‘Umar b. al-Khattab. I asked him: What is that, O Abu Hafs? He replied: He (the Prophet) had said before you came: If any one of you performs ablution, and does it well, and when he finishes the ablution, he utters the words : I bear witness that there is no deity except Allah, He has no associate, and I bear witness that Muhammad is His Servant and His Messenger, all the eight doors of Paradise will be opened for him; he may enter (through) any of them.
Mu’awiyah said: Rabi’ah b. Yazid narrated this tradition to me from Abu Idris on the authority of ‘Uqbah b. ‘Amir.