The eminent Muslim scholar and renowned Da`iyah, Sheikh `Abdel Khaliq Hasan Ash-Shareef, states: “It should be clear that one who is praying behind an Imam is to keep quiet while the Imam is reciting aloud, as Allah says in the Glorious Qur’an: “When the Qur’an is recited, listen and remain silent that you may attain mercy.” (Al-A`raaf: 204)

However, when the imam gives silent recitation, one has to move the lips with the recitation, bearing into consideration that reciting or reading indicates moving the lips with the recitation.”

If, however, for some reason a person cannot move his lips, then his salah is valid provided at least his mind and heart are fully participating in the act of salah. But this exception is granted only to those who are prevented from using their tongue or lips, and not to those who have no excuse.