سنن ابن ماجه

It was narrated that Abu Hurairah said: “The Messenger of Allah (ﷺ) said to a man: ‘What do you say during your Salat?’ He said: ‘The Tashah-hud, then I ask Allah for Paradise, and I seek refuge with Him from Hell, but I do not understand what you and Mu’adh murmur (during Salat). He said: ‘Our murmuring revolves around the same things.’”