Bismillah,
قُلِ ٱدْعُواْ ٱللَّهَ أَوِ ٱدْعُواْ ٱلرَّحْمَـٰنَ أَيّاً مَّا تَدْعُواْ فَلَهُ ٱلأَسْمَآءَ ٱلْحُسْنَىٰ وَلاَ تَجْهَرْ بِصَلاَتِكَ وَلاَ تُخَافِتْ بِهَا وَٱبْتَغِ بَيْنَ ذٰلِكَ سَبِيلاً
Translation: Say: "Call upon Allah, or call upon Rahman: by whatever name ye call upon Him, (it is well): for to Him belong the Most Beautiful Names. Neither speak thy Prayer aloud, nor speak it in a low tone, but seek a middle course between."
When the Prophet, peace and blessings of Allah be upon him, used to call Allah, may He be exalted, he used to say: O Allah, O Compassionate One. So the disbelievers would think why the Prophet would forbid them from worshipping multiple Gods when he would call upon two Gods - "Allah" and the "Compassionate One". This is why the verse was revealed: "Call upon Allah, or call upon Rahman (compassionate one)". This means, call him by any of his 99 names, as narrated in a saheeh hadeeth from Tirmidhi.