Allah says:

[قُلْ]

(Say) O Muhammad, to these idolators who deny that Allah possesses the attribute of mercy and refuse to call Him Ar-Rahman,

[ادْعُواْ اللَّهَ أَوِ ادْعُواْ الرَّحْمَـنَ أَيًّا مَّا تَدْعُواْ فَلَهُ الاٌّسْمَآءَ الْحُسْنَى]

("Invoke Allah or invoke Ar-Rahman (the Most Gracious), by whatever name you invoke Him (it is the same), for to Him belong the Best Names.) meaning, there is no difference between calling on Him as Allah or calling on Him as Ar-Rahman, because He has the Most Beautiful Names, as He says:

[هُوَ اللَّهُ الَّذِى لاَ إِلَـهَ إِلاَّ هُوَ عَالِمُ الْغَيْبِ وَالشَّهَـدَةِ هُوَ الرَّحْمَـنُ الرَّحِيمُ ]

(He is Allah, beside Whom none has the right to be worshipped but He the All-Knower of the unseen and the seen. He is the Most Gracious, the Most Merciful.) (59:22) Until His saying;

[لَهُ الاٌّسْمَآءُ الْحُسْنَى يُسَبِّحُ لَهُ مَا فِى السَّمَـوَتِ وَالاٌّرْضِ]

(To Him belong the Best Names. All that is in the heavens and the earth glorify Him.) [59:24] Makhul reported that one of the idolators heard the Prophet saying when he was prostrating: "O Most Gracious, O Most Merciful.'' The idolator said, he claims to pray to One, but he is praying to two! Then Allah revealed this Ayah. This was also narrated from Ibn `Abbas, and by Ibn Jarir.