The Muwatta, the well-trodden path, is a book of some seventeen hundred reports arranged by legal topic: purity, prayer, alms, marriage, sales, oaths, inheritance. What makes it unlike the later hadith collections is that Malik does not confine himself to prophetic reports. He gives the rulings of the Companions and of the Medinan authorities who followed them, he gives his own opinion, and at intervals he says that a matter is settled by amal, the practice, meaning the continuous habit of Medina.
That last category is his distinctive contribution and the point on which the school was later attacked. The argument is that a city whose population had learned its religion directly and had gone on doing the same things without interruption carried a form of evidence that no chain of individual reporters could match. A defective chain matters less if the whole community has been doing the thing all along.
Critics answered that this turned local custom into revelation and that the practice of one city could not bind the rest. The exchange is worth reading because it is one of the earliest arguments in Islamic law about what counts as evidence at all, rather than about the answer to any particular question, and because both positions are still recognisable in the way the schools differ.