Surah Al-Isra 17:110

قُلِ ٱدْعُوا۟ ٱللَّهَ أَوِ ٱدْعُوا۟ ٱلرَّحْمَـٰنَ ۖ أَيًّا مَّا تَدْعُوا۟ فَلَهُ ٱلْأَسْمَآءُ ٱلْحُسْنَىٰ ۚ وَلَا تَجْهَرْ بِصَلَاتِكَ وَلَا تُخَافِتْ بِهَا وَٱبْتَغِ بَيْنَ ذَٰلِكَ سَبِيلًا

Say, "Call upon Allāh or call upon the Most Merciful [ar-Raḥmān]. Whichever [name] you call - to Him belong the best names." And do not recite [too] loudly in your prayer or [too] quietly but seek between that an [intermediate] way.

Translation: Saheeh International

Tafsir

The Prophet s would often say ‘O God O Compassionate One’. So they said ‘He Muhammad (s) forbids us to worship more than one god and yet he calls upon another god together with Him God’. …

Excerpt from Tafsir al-Jalalayn, commentary on 17:110, via Quran.com. The reader has the full commentary and 9 more tafsirs.