প্রশ্নোত্তর

Assalamu alaikum Prof. I have two questions:
1. Is it mandatory to follow anyone madhab? Please, Prof. explain or suggest to me one authentic book on madhab.
2. Last time I have seen some youtube video one alem (popular alem) said to another alem (popular alem) is kafir, munafiq or agent of Yehudi, Christian etc. Now, I dont understand who is really follow sunnah. So, please suggest to me some authentic books.