Al-Nawawi (may Allaah have mercy on him) said:
Dhikr may be in the heart, or on the tongue, but the best is that which is both in the heart and on the tongue. If it is limited to just one of them, then the heart is better. End quote from al-Adhkaar (p. 20).
Ibn al-Qayyim (may Allaah have mercy on him) said:
The types of dhikr may be in the heart and on the tongue sometimes, which is the best of dhikr, or they may be in the heart only sometimes, which is the second level, and on the tongue sometimes, which is the third level. The best of dhikr is that on which both the heart and tongue are focused, and dhikr in the heart only is better than dhikr on the tongue only, because dhikr in the heart increases knowledge, provokes love, increases shyness before Allaah, promotes fear of Allaah, calls one to draw closer to Him, and it prevents one from falling short in acts of worship and being heedless about sins and bad deeds. Dhikr on the tongue only does not give any of these results and it is of little benefit.
End quote from al-Waabil al-Sayyib min al-Kalim al-Tayyib (p. 120). See also Madaarij al-Saalikeen (2/420)