´`Abdullah bin Mas`ud said:` "It is from the Sunnah to say the Tashahhud quietly"
Explanation & Benefits
Shaykh Dr. Abdur Rahman Freywai
1:
When a Companion says, “It is from the Sunnah to do such-and-such,” or, “The Sunnah is such-and-such,” then according to the majority, this is considered to have the ruling of being marfu‘ (attributed directly to the Prophet sallallahu alayhi wa sallam).
Source: Sunan al-Tirmidhi – Majlis ‘Ilmi Dar al-Da‘wah, New Delhi Edition, Page: 291