Is reciting with the heart sufficient in prayer?
Machine translationRead the Arabic original
The questioner : The questioner asks: Is it obligatory to move the tongue with the Qur'an in prayer, or is it sufficient to recite it in the heart?
The Shaykh : The Qur'an, or rather the recitation, must be with the tongue. If a person recites in his heart during prayer, that does not suffice for him. Likewise, all other dhikr (remembrance of Allah) is the same; the dhikr is not permissible with the heart alone. One must move his tongue and lips with it, such as "Subhan Rabbi al-Azim" (Glory be to my Lord, the Exalted) and "Allahu Akbar" (Allah is the Greatest), and the like. One must pronounce them because they are words, and they are not realized except by moving the tongue and lips.
The Shaykh : The Qur'an, or rather the recitation, must be with the tongue. If a person recites in his heart during prayer, that does not suffice for him. Likewise, all other dhikr (remembrance of Allah) is the same; the dhikr is not permissible with the heart alone. One must move his tongue and lips with it, such as "Subhan Rabbi al-Azim" (Glory be to my Lord, the Exalted) and "Allahu Akbar" (Allah is the Greatest), and the like. One must pronounce them because they are words, and they are not realized except by moving the tongue and lips.
Source
دروس وفتاوى المسجد الحرام لعام 1402هـ - 3 — binothaimeen.net (8:46)
The Arabic text is a machine transcription of the audio recording.