Is the silent remembrance of God (Dhikr Qalbi) without uttering it, an act of worship of the heart or of the tongue?

Dhikr (remembrance of God) by the heart is an act of worship of the heart, not an act of verbal expression unless the tongue pronounces it. The best dhikr is that which the heart and tongue are in agreement. Dhikr of the heart is superior to dhikr of the tongue alone because it yields knowledge, love, fear, and mindfulness, whereas the fruit of dhikr of the tongue alone is weak.