Is The Koran the Only Sacred Source of Law for Muslim?
The Koran – in spite of being the ultimate authority and source for Islamic law and doctrine – is not the only source of Islamic law and doctrine. There are other sources other than it that Muslims rely upon. Some of those sources are agreed upon by all Muslims, while others are not. For instance,
The second source, the Sunnah, is agreed upon by all Muslims, both Sunnis and Shiites. However, Sunnis and Shiites differ about the types of reports that constitute a genuine Sunnah and those that do not.
Scholarly Consensus (Ijma’), another source, is accepted by Sunnis, while Shiites reject it.
Legal Analogy (Qiyaas) is another source of Islamic law and doctrine that almost all Muslims accept as being valid through mainstream Shiites (and some Sunnis) object to it as well.
Other than these, there are a number of other sources upon which there is no unanimous agreement about their validity.
But these four are the primary of all Islamic sources. And they are the sources over which Muslims have the least amount of discrepancy.