The Prophet (peace be upon him) said, “Pray as you saw me praying.”  (Al-Dhahabi, Al-Busiri and others). So, we are obligated to perform our prayers as he prayed; we are not allowed to make up on our ways of doing it. Allah sent His messenger to be obeyed and this obedience is due primarily in all matters of religion, including all acts of worship such as prayer, fasting, charity, and pilgrimage. In regards to such issues, we are to follow him and emulate his examples.

 

Therefore, he warned us against innovations in religious matters: “whoever innovates a new way in religion it ought to be rejected.” (Abu Dawud and others)

 

Now let me come straight to your question:

 

If you mean to ask you want to make your prescribed supplications without words by meditating in your mind that is not acceptable as long as you are a person who can articulate; if however, you have speech impediment then you are allowed to say it in your mind without words.

 

Having said this, I should also add two points:

 

Reading the supplications with your tongue does not in any way mean saying them aloud; it only means moving your lips in such a manner that you can hear.

 

After having uttered the essential words of the Qur’an and Tasbih and essential supplications that are prescribed in prayer, you may make any personal supplication of your choice in your mind without moving your lips; instead, you can do so in your mind.

 

Outside the prayer, you may engage in silent dhikr or supplications in your mind without uttering words.

 

Almighty Allah knows best.