Is it required to conceal (recite silently) the Tashahhud during prayer?

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It is a Sunnah to conceal the Tashahhud because the Prophet (PBUH) never recited it aloud; had he done so, it would have been transmitted like the recitation of the Quran was transmitted. Ibn Mas'ud narrates that concealing the Tashahhud is from the Sunnah. Furthermore, since it is a remembrance that is not the Quran and does not transition the worshipper from one pillar to another, it is recommended to be concealed, similar to Tasbih (glorification). There is no known disagreement on this point.