How are religious texts preserved from distortion, and what are the controls followed for that? Is their preservation achieved through ijāzah (authorization to transmit) and samāʿ (hearing from a teacher), or by comparing copies and manuscripts, or by the ijtihād (independent reasoning) of verifiers in correction? Are there books that address this topic?

In the past, authors would dictate their books to scribes. After the books were written, they would be presented to the authors for their approval. Sometimes, the author would write the book himself, and then scribes would copy it, recording their names and the date of copying. In addition, there were "listening sessions" (samāʿāt) where a copy would be signed by a renowned scholar after it was read aloud. Thus, works were transmitted in a continuous chain across generations and settled in libraries. Then came the role of manuscript authenticators (muḥaqqiqūn) to verify the manuscript and attribute it to its author, which is a specialized field in itself. The foundational texts of the Islamic sciences have been preserved in both written and continuous, unbroken chains of transmission, which reassures one regarding their preservation. Furthermore, Allah's promise to preserve the Qur’an and the Sunnah guarantees the preservation of the meanings and rulings of Islamic law in general, even if some distortion were to occur in a particular book. There are many works dedicated to the science of manuscript authentication.