Is it a condition for the validity of dhikr or du'a for a sound to be produced when repeating it, or is moving the tongue sufficient?

Dhikr (remembrance of Allah) and du'a (supplication) can be performed with the heart, or the tongue, or both, the latter being the most complete. Scholars have differed regarding dhikr with the tongue: whether it requires one to hear oneself or if merely moving the tongue and lips suffices. The more prevalent view is that moving the tongue and lips is sufficient. The references for this issue are books of Tafsir (exegesis of the Quran), commentaries on hadith pertaining to dhikr, and books of adhkar (collections of supplications and remembrances).