Abu Dawud Book 2, Hadith Number 1488.

Chapter : Supplications.

Narated By Buraydah ibn al-Hasib : The Apostle of Allah (PBUH) heard a man saying: O Allah, I ask Thee, I bear witness that there is no god but Thou, the One, He to Whom men repair, Who has not begotten, and has not been begotten, and to Whom no one is equal, and he said: You have supplicated Allah using His Greatest Name, when asked with this name He gives, and when supplicated by this name he answers.