What is the ruling on seeking forgiveness with the heart without uttering it with the tongue?

Seeking forgiveness (istighfar) is one of the virtuous forms of remembrance (dhikr). It is permissible to do it with the heart without uttering it, and one is rewarded for it. However, it is better for it to be with both the heart and the tongue. Thus, remembrance can be with both the heart and the tongue (the best form of dhikr), or with the heart alone (the second degree), or with the tongue alone (the third degree).