Enjoining what is good (al-ma’roof, i.e. Islamic Monotheism and all that Islam has ordained) and forbidding what is evil (al-munkar, polytheism, disbelief and all that Islam has forbidden) is one of the most important Islamic duties.

From the Holy Quran:

And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful.
[Aal ‘Imraan 3:104]

You are the best nation produced [as an example] for mankind. You enjoin what is right and forbid what is wrong and believe in Allah . If only the People of the Scripture had believed, it would have been better for them. Among them are believers, but most of them are defiantly disobedient.
[Aal ‘Imraan 3:110]

Cursed were those who disbelieved among the Children of Israel by the tongue of David and of Jesus, the son of Mary. That was because they disobeyed and [habitually] transgressed.

They used not to prevent one another from wrongdoing that they did. How wretched was that which they were doing.
[al-Maa’idah 5:78–79]

Luqmaan said to his son:
O my son, establish prayer, enjoin what is right, forbid what is wrong, and be patient over what befalls you. Indeed, [all] that is of the matters [requiring] determination.
[Luqmaan 31:17]

[And they are] those who, if We give them authority in the land, establish prayer and give zakah and enjoin what is right and forbid what is wrong. And to Allah belongs the outcome of [all] matters.
[al-Hajj 22:41]

From Hadith:

Abu Sa’eed said:
“I heard the Messenger of Allah [SAW] say:
‘Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart- and that is the weakest of Faith.'”

Sunan an-Nasa’i 5008 [Sunnah.com/QuranX/iHadis (Bangla)]
In-book reference: Book 47, Hadith 24
English/Dar-us-Salam translation: Vol. 6, Book 47, Hadith 5011

Commentary:
From hadithaday.org
From hadithcommentary.wordpress.com

Same or similar Hadith in other books can be found over this note.

For further reading:
Enjoining what is good and forbidding what is evil
Enjoining what is good and forbidding what is evil, and the verse, “Take care of your ownselves”
Should he denounce evil and remind people to stop it every time it happens?
Is he sinning if he sees an evil action and does not denounce it?