Jami` at-Tirmidhi 291
جامع الترمذي#291SahihAbdullah bin Mas`ud said:"It is from the Sunnah to say the Tashahhud quietly
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 2:143