Al-e-Imran (The House of Imran) Sura 3: Verse 104
"Let there arise out of you a band of people inviting all to what is good, enjoining what is right, and forbidding what is wrong. Such are the ones who shall prosper."
This is the commandment to the Muslim community to establish a system that is essentially based on holding fast to Allah's rope or covenant and is a necessary prerequisite to help people to abide by its demands. For this purpose, the Muslims are instructed to appoint a group from among them to enjoin maruf or good, or the noble conventions of society, and to forbid munkar or evil as defined by the Islamic Shariah. The way the imperatives — enjoining and forbidding — are used concerning good and evil clearly indicates that this task is to be carried out not by mere verbal advice and admonition, but must also be enforced by law and authority. This is impossible without political power vested in such a group. If the purpose were to accomplish the task of enjoining good and forbidding evil by advice and propagation of dawah only, the words yaduna ila-l khayr (inviting to good) should have sufficed and there was no need to add ya muruna bi-l maruf (enjoining or commanding what is right).
It was indeed in compliance with this Divine commandment that the first thing that the Muslims attended to was the establishment of a caliphate on the pattern of the Prophet's model of governance. The primary purpose of this institution was to monitor and stand guard so that there should be no deviation from the ideal of itisam billah or holding fast to Allah. In terms of principles, it adopted three methods in order to realise this goal: inviting people to good, enjoining what is right and just and forbidding what is wrong and evil. Out of these three sprang all other departments during the rightly guided caliphate. These were used as a means of meeting all the internal and external responsibilities of the Muslim ummah.
"Pondering Over The Qur'an: Surah Ali Imran" - Amin Ahsan Islahi