Allamah Ibn Muflih al-Hanbali (rahimahullah) wrote:

Chapter: It is necessary to disapprove of unlegislated actions, even if many people do them

It should be known that numerous matters are practiced by a large number of people contrary to the legal (shar’i) command, and such practices become widespread among them. Many others then follow these people in their actions. It is obligatory upon the one who has knowledge to oppose them in this, both in word and deed. He should not be discouraged by being alone or having few companions.

Shaykh Muhyi al-Din al-Nawawi (rahimahullah) said: “One should not be deceived by the abundance of people doing what is forbidden, from those who do not observe these proper manners.” Also, follow the saying of the noble leader, al-Fudayl ibn ‘Iyad (rahimahullah): “Do not feel lonely on the path of guidance due to the fewness of its followers, and do not be deceived by the large number of those who are ruined.”

Abu al-Wafa’ Ibn ‘Aqil (rahimahullah) said in ‘al-Funun’: “Whoever’s belief is based on proof will not change according to people’s states.” [Allah says:]

﴾أَفَإِن مَّاتَ أَوْ قُتِلَ انقَلَبْتُمْ عَلَىٰ أَعْقَابِكُمْ﴿

“If he (the Prophet) dies or is killed, will you then turn back on your heels?” (Ali ‘Imran 3:144).

Al-Siddiq (Abu Bakr – radiyallahu ‘anhu) was among those who remained firm despite the changing circumstances, and he was not swayed in every situation where others stumbled.” Until he said: “A person might be a Muslim until his living situation becomes difficult [and then he disbelieves]. However, our religion is based on [being patient upon] the hardship of this world and seeking success in the Hereafter. Whoever seeks worldly gains through it is mistaken.”

Source: al-Adab al-Shar‘iyyah (1/263)