"Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment"

[al-Maa'idah 5:2]

"Those among the Children of Israel who disbelieved were cursed by the tongue of Dawood (David) and 'Eesa (Jesus), son of Maryam (Mary). That was because they disobeyed (Allaah and the Messengers) and were ever transgressing beyond bounds.

They used not to forbid one another from Al-Munkar (wrong, evildoing, sins, polytheism, disbelief) which they committed. Vile indeed was what they used to do"

[al-Maa'idah 5:78, 79]

And the Prophet (peace and blessings of Allaah be upon him) said: "Whoever calls others to right guidance will have a reward like that of those who follow him, without that detracting from their reward in the slightest, and whoever calls others to misguidance will have a burden of sin like that of those who follow him without that detracting from their sins in the slightest." Narrated by Muslim, 2674.

And he 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 hating it and feeling that it is wrong), but this is the weakest of faith." Narrated by Muslim, 49.

Shaykh Ibn Baaz (may Allaah have mercy on him) said: "Denouncing in one's heart is obligatory upon everyone, which means hating and despising the evil and separating from those who do it if one is unable to change it with one's hand or tongue, because Allaah says (interpretation of the meaning):

'And when you (Muhammad) see those who engage in a false conversation about Our Verses (of the Qur'aan) by mocking at them, stay away from them till they turn to another topic. And if Shaytaan (Satan) causes you to forget, then after the remembrance sit not you in the company of those people who are the Zaalimoon (polytheists and wrongdoers)'

[al-An'aam 6:68]."