Sunan Abi Dawud 986
سنن أبي داود#986SahihNarrated Abdullah ibn Mas'ud: It pertains to the sunnah to utter the tashahhud quietly
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiSahih
In-book reference: 2:597