It pertains to the sunnah to utter the tashahhud quietly.
Sunan Abi Dawud Book 2, Hadith 597
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The Messenger of Allah (ﷺ) entered the mosque and saw a man who had finished his prayer, and was reciting the tashahhud saying: O Allah, I ask you, O Allah, the One, the eternally besought of all, He begetteth not, nor was He begotten, and there is none comparable unto Him, that you may forgive me my sins, you are Most Forgiving, Merciful.
He (the Prophet) said: He was forgiven (repeating three times.)
Sunan Abi Dawud Book 2, Hadith 596 Next Hadith → ‘Abd Allah b. ‘Umar saw me playing with pebbles during prayer. When he finished his prayer, he forbade me (to do so) and said: Do as the Apostle (ﷺ) used to do. I asked him: How would the Messenger of Allah(ﷺ) do? He said: When he sat during the prayer (for reciting the tashahhud), he placed his right hand on his right thigh, and clenched all his fingers, and pointed with the finger which is adjacent to the thumb, and he placed his left hand on his left thigh.
Sunan Abi Dawud Book 2, Hadith 598
He (the Prophet) said: He was forgiven (repeating three times.)
Sunan Abi Dawud Book 2, Hadith 596 Next Hadith → ‘Abd Allah b. ‘Umar saw me playing with pebbles during prayer. When he finished his prayer, he forbade me (to do so) and said: Do as the Apostle (ﷺ) used to do. I asked him: How would the Messenger of Allah(ﷺ) do? He said: When he sat during the prayer (for reciting the tashahhud), he placed his right hand on his right thigh, and clenched all his fingers, and pointed with the finger which is adjacent to the thumb, and he placed his left hand on his left thigh.
Sunan Abi Dawud Book 2, Hadith 598