Hadith 706
Sayyiduna Abdullah (may Allah be pleased with him) narrates that the Sunnah method is that you recite the Tashahhud in a low voice.
Hadith 707
Lady Aisha (may Allah be pleased with her) narrates that this verse was revealed regarding Tashahhud, « وَلَا تَجْهَرْ بِصَلَاتِكَ وَلَا تُخَافِتْ بِهَا » [ سورة الإسراء ] "And do not recite your prayer loudly, nor in a very low voice."