Peace be upon you. According to Ayatollah Khamenei (may Allah protect him), during the Tashahhud in prayer, one should only recite the matters mentioned in the practical jurisprudence texts, which are the two testimonies. It is not permissible to add anything beyond that, even if the words are true and correct. Source: Special Inquiry. Additionally, Ayatollah Sistani (may Allah protect him) states that it is strongly recommended to refrain from it. Source: Official website. May you be successful in all good.