Bilal - it's bit of a 'generalisation' to suggest that professors stop researching. Some might, for sure, but I would argue that most will continue - perhaps to a varying degree prior to their professorship. With professorships often comes more responsibility i.e. more university community and administrative roles (Committees, Head of Schools etc) and other demands which, inevitably, restricts and reduces the time available to conduct research. On the other hand, it can have advantages that enhance research opportunities i.e. more invitations from research teams, more theses supervisions etc. Personally, I do not know of any professorial colleagues who do not research to some degree or other. All the universities that I have worked at have it as part of the contractual agreement for the post. The only exception that I can think of is professors who have a senior leadership management role - whereby their contractual business is purely university administration.