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.
Say, O Messenger, to those who criticise you for supplicating by saying, ‘O Allah, O Merciful’: Allah and the Merciful are names of His, so supplicate Him with any of them or other names of His. For Him are all beautiful names, and these two are amongst them, so supplicate Him through these two or other beautiful names of His. And do not recite loudly in your prayer in case the idolaters hear you and do not recite so quietly that the believers cannot hear you, but rather choose a middle path between the two.