مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 918
Prayer
The Tashahhud - Section 3
وَعَنِ ابْنِ مَسْعُودٍ كَانَ يَقُولُ: مِنَ السُّنَّةِ إِخْفَاءُ التَّشَهُّدِ. رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَقَالَ: هَذَا حَدِيثٌ حَسَنٌ غَرِيبٌ
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.
Mishkat al-Masabih, 918صَحِيح (الألباني)