First of all, we’d like to clarify that there are two types of dhikr (remembrance of Allah): dhikr with the tongue such as reciting Qur’an and du`a’ (supplication), which is encouraged in Shari`ah, and dhikr in the heart, which means thinking of Allah and His greatness and power or reciting the Qur’an in one’s heart. Both types are acceptable and encouraged in Islam.
The eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states: “There is no doubt that any degree or form of dhikr (remembrance of Allah) is acceptable, praiseworthy and rewardable in Islam. The mere contemplation with the heart or making dhikr of Allah in your heart is a form of dhikr that is valid and acceptable according to the Islamic Shari`ah. Occupying the heart with dhikr or contemplation on Allah’s creation is better than occupying it with something else. However, there is fear that with complete silent dhikr, one may get distracted easily and does not continue for a long time, unlike dhikr with the tongue even with a low voice.
It is noticeable that thoughts of the heart are conflicting with each other. So, it would be preferable if you make dhikr with your tongue even with a low voice to avoid such distraction. If you cannot, there is nothing wrong with the complete silent dhikr.”