(187) باب إِخْفَاءِ التَّشَهُّدِ
Narrated Abdullah ibn Mas'ud:
It pertains to the sunnah to utter the tashahhud quietly. (ID)
Reference (ID): Sunan Abi Dawud 986 (ID)
In-book reference (ID): Book 2, Hadith 597 (ID)
English translation (ID): Book 3, Hadith 981 (ID)