For Muslims, the Hadith are EXTREMELY important for living the Muslim life. Yet, different Muslims believe in different books of the Hadith. In fact, some Muslims will take a book and claim that portions of it are accurate to the instructions of Muhammad, but some are not.
The “science” behind their reasoning is that the Hadith need to be authentic in their Isnad (chain or line of authority) and Matn (text). In other words, not only are the words themselves important, but also who said they heard it from Muhammad is just as important to Muslims.
This is the reason why different sects within Islam disagree on the books of Hadith: they each believe different people are trustworthy and others are not (differences in Isnad).
Next, let’s talk about Shari’a. It’s the law of Islam. While directly translated as “the path to water”, Shari’a is basically what God wants people to do in life. Hand in hand with Shari’a is Fiqh, or Islamic jurisprudence. This is how people figure out what Shari’a is. This is because God did not reveal Shari’a in a book for Muslims. It’s done by combining understandings of the Qur’an and the Hadith through the process of Fiqh in order to derive Shari’a. Other components of Shari’a – in addition the Qur’an and Hadith – are Ijma (the consensus of scholarship), and Qiyas (reasoning).
Here’s an example of how all this plays out. If a Muslim is presented with the question of whether or not they can marry someone, the first place they will go to find the answer is the Qur’an. Next, they will turn to the Hadith. Third, they will look to see what the governing scholarship says. And finally, they will use their own reasoning.
This is largely why Muslims are upset by wide-sweeping news reports of what Shari’a law decrees, since there are so many different interpretations and avenues of reaching an interpretation. And, traditionally speaking, there are four main schools of thought within Sunni Islam, constituting where Sunni Muslims turn to for guidance. These four are: Shafi, Hanbali, Hanafi, and Maliki. And while there are distinctions between these four, there are also lots of overlap. One example is: if you leave Islam, you can be killed for your apostasy.
All this to say, almost all Muslims believe and follow Islam according to what they are told. There is not very much individual exegesis, or critical interpretation of Scripture, done within Islam. Muslims go to their authorities and ask them, “What should I do in this situation? Give me a decree or decision on what I ought to do.”
As you can see, since Shari’a is not monolithic, there is a lot of complexity within Islam.