Allaah has described the believers as denouncing evil and as enjoining what is good. He says (interpretation of the meaning):
“The believers, men and women, are Awliyaa’ (helpers, supporters, friends, protectors) of one another; they enjoin (on the people) Al‑Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do), and forbid (people) from Al‑Munkar (i.e. polytheism and disbelief of all kinds, and all that Islam has forbidden)”
“Let there arise out of you a group of people inviting to all that is good (Islam), enjoining Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden)”
[Aal ‘Imraan 3:104]
“You (true believers in Islamic Monotheism, and real followers of Prophet Muhammad and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al‑Munkar (polytheism, disbelief and all that Islam has forbidden)”
[Aal ‘Imraan 3:110]
There are many verses which speak of enjoining what is good and forbidding what is evil; that is because of the importance of this issue and the great need that exists for it.
According to a saheeh hadeeth, the Prophet (peace and blessings of Allaah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart [by at least hating it and believing that it is wrong], and that is the weakest of faith.” Narrated by Muslim in his Saheeh.
Changing evil by one's hand should be done by those who are able to do that, such as those in positions of authority and the bodies in charge of such matters in the areas over which they have been appointed, the ruler in the area over which he has been appointed, the judge in the area over which he has been appointed, and by a man in his own house with his children and the members of his household, as much as he can.
But if he is not able to do that, or if changing it with his hand will lead to fitnah (tribulation) and conflict, and cause more harm than good, then he should not change it with his hand, rather he should change it with his tongue, by speaking out. That will be sufficient lest changing it with his hand will lead to a worse evil that that which he is seeking to change. This was stated by the scholars.
In the case of changing evil with his tongue, it is sufficient for him to say, “O my brother, fear Allaah, this is not permitted, you should stop this, you should do such and such” and other good words spoken in a proper manner.
After the tongue comes the heart, i.e., hating the evil in one's heart, showing that one hates it, and not sitting with those who do it. All of these come under the heading of changing it with one's heart. And Allaah is the Source of strength.