When did Muslims start compiling Hadiths? What were the criteria to judge or acertain the authencity of hadiths?

 The compilation of Hadiths, or sayings and actions of the Islamic Prophet Muhammad, began during his lifetime and continued after his death. The process of collecting and documenting Hadiths started in the 7th century and continued for several generations.

To ascertain the authenticity of Hadiths, scholars developed a set of criteria known as the science of Hadith criticism (ʻIlm al-Rijāl). These criteria are used to evaluate the chain of narrators (isnads) and the content (matn) of the Hadiths. Some of the key criteria include:

1. Chain of Narrators: Scholars examined the reliability and integrity of each narrator in the chain, considering factors such as their memory, character, and credibility. This process involved examining the biographical details of each narrator to ensure they were trustworthy and had a strong memory.

2. Continuity of Narrators: Scholars looked for a continuous chain of narrators, ensuring that there were no gaps or missing links in the transmission of the Hadith.

3. Multiple Chains: If a Hadith was narrated through multiple independent chains of narrators, it increased its authenticity and reliability.

4. Absence of Contradictions: Scholars checked for any contradictions or inconsistencies within the Hadith itself or between different Hadiths. If a Hadith contradicted established principles of Islam or other authentic Hadiths, it would be considered weak.

5. Preservation of Text: Scholars analyzed the text of the Hadith to ensure it was coherent, consistent, and aligned with the teachings of Islam.

These criteria were developed and refined by eminent Hadith scholars over time to establish a rigorous methodology for determining the authenticity and reliability of Hadiths. This process resulted in the classification of Hadiths into different categories based on their level of authenticity, such as Sahih (authentic), Hasan (good), or Da'if (weak).