Narrated by Abū Ḥamzah Anas ibn Mālik (may Allāh be pleased with him), the servant of the Messenger of Allāh (ﷺ), that the Prophet (ﷺ) said: ‘None of you truly believes until he loves for his brother what he loves for himself.’ Narrated by al-Bukhārī and Muslim.¹

Imām Ibn Rajab [d. 795 AH] explains:

All believers should love for their believing brothers what they love for themselves, and hate for them whatever they hate for themselves. Thus, if anyone should observe a deficiency in the dīn of his believing brother, then he should strive to correct him. Some of the pious predecessors used to say: ‘The people who truly love for Allāh observe others with the light [guidance] of Allāh, they show compassion upon the one who engages in disobedient actions, they detest their actions while showing mercy to them such that they can exhort them; turning them away from their actions [of disobedience]. They are empathetic towards the believers, seeking that their bodies should never touch the Fire.’

Endnotes: 

1. Authentic: Narrated by al-Bukhārī:13 and Muslim:45.

Source: Jāmiʿ al-ʿUlūm: 332
Translated by: Riyāḍ al-Kanadī