Sahih Muslim #45

مَنْ رَأَى مِنْكُمْ مُنْكَرًا فَلْيُغَيِّرْهُ بِيَدِهِ فَإِنْ لَمْ يَسْتَطِعْ فَبِلِسَانِهِ فَإِنْ لَمْ يَسْتَطِعْ فَبِقَلْبِهِ وَذَلِكَ أَضْعَفُ الإِيمَانِ

The Messenger of Allah (ﷺ) said: 'He who amongst you sees something wrong should change it with his hand; if he is unable to do so, then with his tongue; if he is unable to do so, then with his heart — and that is the weakest form of faith.'