Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned:
It is generally witnessed that many of those who do not subscribe to the following of any mazhab harbour ill thoughts and bad opinions regarding those who follow an Imaam. This quality of thinking good of oneself and harbouring ill thoughts regarding others is a root cause of all evils and wrongs. Thereafter, this evil quality within such a person progresses to the point where it causes him to run down followers of a mazhab and criticize them. Even if such a person is not guilty of these wrongs, then too treading upon such a path (i.e. remaining unrestricted and uncommitted to the following of one mazhab) is an extremely dangerous path to tread on in one’s Deen. (Malfoozaat Hakeemul Ummat 8/274)
Source: Ihyaauddeen.co.za