The Tashahhud - Section 3 Mishkat al-Masabih 918
Arabic hadith text
وَعَنِ ابْنِ مَسْعُودٍ كَانَ يَقُولُ: مِنَ السُّنَّةِ إِخْفَاءُ التَّشَهُّدِ. رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَقَالَ: هَذَا حَدِيثٌ حَسَنٌ غَرِيبٌ
Translation
Ibn Mas'ud used to say, “It pertains to the sunna to repeat the tashahhud quietly.” Abu Dawud and Tirmidhi transmitted it, and Tirmidhi said this is a hasan gharib tradition.