Surah 17

Surah 17:110

"Say: ‘Call on God or call on the Merciful – whichever you call on, to Him (belong) the best names.’ And do not be loud in your prayer, nor silent in it, but seek a way between that."

17.110 – whichever you call on, to Him (belong) the best names
Here Allāh and Raḥmān seem to be equally appropriate names for God. The verse adds that God has many beautiful names.

- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam