Is it permissible to supplicate silently without sound to avoid the whispers of Satan?

Supplication is not valid by the heart alone; rather, it must be uttered verbally. Satan or anyone else possesses no power to benefit or harm except with Allah's permission, and reliance on Him suffices the servant. Allah the Almighty said: ﴿Is not Allah sufficient for His Servant?﴾, and ﴿And if Allah should touch you with adversity, there is no remover of it except Him.﴾, and ﴿And whoever relies upon Allah - then He is sufficient for him.﴾ The Prophet, peace and blessings be upon him, said: «And know that if the nation were to gather together to benefit you with something, they would not benefit you except with something Allah has already written for you. And if they were to gather together to harm you with something, they would not harm you except with something Allah has already written against you.»