The defining mark of the Ahlus Sunnah wal Jama’at is that, together with accepting the Holy Qur’an as the foundation of religion, they understand the Sunnah of the Messenger of Allah (sallallahu alaihi wa sallam), that is to say his statements and his practice, to be its exposition and the elaboration of what it sets out in principle, and that whatever has not been explained in the Holy Qur’an itself is then to be found in the Sunnah. They therefore regard the Sunnah as also being an intrinsic part of religion and something that has to be followed.
Then, together with accepting the Book of Allah and the Sunnah of the Messenger(sallallahu alaihi wa sallam) in this way, they also regard the Sahabah(RA) as the measure by which these things are to be understood. That is to say, whatever they understood to be the intent of anything in the Book of Allah or the Sunnah, and whatever rulings they were all agreed on also have to be followed, and no Muslim has the right to hold any contrary opinion on the rulings and decisions on which they were all in agreement.
In the view of the Ahlus Sunnah wal Jama’at the ijma(consensus) of the Sahabah (RA) on any matter of religion means that the matter becomes definitive, and to disagree with it is then error, because without any doubt the Sahabah(RA) knew better than anyone else the environment, the atmosphere, the conditions, and the language in which the Din came.
Then, they acquired the Din direct from the Messenger of Allah (sallallahu alaihi wa sallam), and had the benefit of his company and his training. Thus, no one can have a better knowledge of religion and understanding of the spirit and the intent of the teaching of the Messenger of Allah than them. So religion is whatever they understood it to be.
In short, the Ahlus Sunnah wal Jama’at regard the consensus of the Sahabah(RA)on any question of religion as being decisive, and in their view there is no room for disagreement with it. So, this is the guiding principle of the Ahlus Sunnah wal Jama’at, and indeed the reason for being called the Ahlus Sunnah wal Jama’at is that, after the Book of Allah, they have attached so much importance in religion to the sunnah and the jama’at of the Sahabah(RA) and have bound themselves by them.
(From Din and Shariat by Moulana Manzur Numani RA)