Lit. support. Technically refer to the chain of transmitters, the list of people who successively narrated a given hadith. The text of the hadith is called matn
Early Muslim writers were not too concerned about the isnad. Later on (about two centuries later), debates about what is lawful and what is not lawful intensified among the varous schools, so that more and more prominence was given to the isnad that led back to Muhammad for rulings. It was important to Muslims that rulings were made by Muhammad himself. Muslim traditionists concerned themself with making sure that the isnad is made up of reliable people. Unfortunately, they had no means of ascertaining if the report was true per se, which led to an explosion of fabricated hadiths. As a result, many hadiths were contradictory.