There are many things in Islam which are not explicitly ordered or mentioned in the Qurâan such as the number of rak’as of the prayers but we act according to the conduct of the Prophet (s). Why? Because, we believe that the Prophet (s) had the right of legislation in Islam. In other words, the Prophet (s) revealed some inward divine orders of the Qurâan and communicated them to the Muslims by interpreting the QurâanÂ and they are obliged to obey his orders. This is mentioned in the Qurâan as it says:
“O you who have faith! Obey Allah and obey the Apostle and those vested with authority among you. And if you dispute concerning anything, refer it to Allah and the Apostle, if you have faith in Allah and the Last Day. That is better and more favorable in outcome.” (4:59)
In the same way, we believe (according to some definite reasons) that our infallible Imams (a) have this right of interpretation of the Qurâan and their orders are the same as the orders of the Prophet (a) without any difference and we should obey their orders and conduct.