Sheikh Rashîd al-Hasan, professor at King Khâlid University
It is not compulsory or even preferable for a person to follow a certain school of thought. A Muslim is not obligated to follow one school in particular. The great jurists who founded these schools of law were only concerned about what was found to be authentic. They warned against innovation and against emulating anyone besides the Prophet (peace be upon him). It was related that all four of them (Abu Hanîfah, Mâlik, al-Shâfi`î, and Ahmad) said: “No one is allowed to use our answers to people’s queries or our speech unless he knows where we have got it from.” Each one of them said “If a hadîth is authentic, then it is my position, whether I cited it or not.”
If a man follows Abu Hanîfah, Mâlik, al-Shâfi`î, or Ahmad, then finds that the opinion of some other school of law is more correct, he should follow that opinion. If he does this, then he is doing what is right and no one can derogate his faith.
It was mentioned by al-Kamâl b. al-Humâm in his book al-Tahrîr wa al-Taqrîr that it is not obligatory to follow a certain school of law because there is no evidence for doing so. Obligation is only set by Allah and His Messenger (peace be upon him).
Source: Islam Today