A Supplication Which Allaah Answers – Saheeh Abee Daawood
From Buraydah (رضي الله عنه) that Allaah’s Messenger (صلى الله عليه و سلم) heard a man saying:
Meaning: “O Allaah: I ask you by the fact that I bear witness that You are Allaah, none has the right to be worshipped except You: The Unique One, the perfect Lord and Master upon Whom all the creation depends. The One Who has no parent and no child, and there is none like Him.”
So he (صلى الله عليه و سلم) said: ‹You have asked Allaah by the name which is such that if He is asked by it He gives, and if He is supplicated to by it He responds.› 
 Saheeh: ‘Saheeh Abee Daawood’ 1493
[Translated by Aboo Talhah Daawood ibn Ronald Burbank rahimahullaah]
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Allaahumma innee as-aluka annee ash-hadu annaka antAllaahu laa ilaaha illaa anta, al-ahadu as-samadu, alladhee lam yalid wa lam yoolad, wa lam yakullahu kufuwan ahad.