On the authority of Abu Hurayrah, may Allaah be pleased with him, said: The Messenger of Allaah, may peace and blessings be upon him, said:
“The believer is the mirror of the believer.” [Abu Daawud 4918 with a good chain]
Sheikh Saalih al-Fouzaan, may Allaah preserve him, explains this hadeeth by saying:
“A mirror is that which you see yourself when you stand in front of it whether the picture is presentable or needs fixing and modification. When a person wishes to leave from his home, he stands in front of the mirror because there may be something that he needs to correct on himself or remove from his clothing or the like. For a person to have a pleasant appearance in front of the people is a good thing, however there is the figurative meaning of mirror that shows you your deficiencies and that is your Muslim brother/sister.
The believer is the mirror of his fellow brother. Your brother knows the mistakes and shortcomings that you may have and therefore informs you of them so accept what he tells you. This is an encouragement to accept what your brother presents to you which entails the beautification of your inward and outward appearance. Also he sees things in you that you can’t see yourself. A person may think he is complete and is mistake-free whereas his sincere and genuine Muslim brother sees that he has mistakes and shortcomings thus he instructs him to rectify them. So do not restrict yourself solely to your own assumptions and considerations but consult your brother and listen to the advice he has to give.
In this hadeeth are two important matters:
1) A person should accept the advice of his sincere brother from that which he sees in him from defects and correct them.
2) It is compulsory upon the Muslim to advise his brother/sister and not remain silent about what he sees in him from defects and the like or the opposite; praising him and being phony with him by relaying to him that which isn’t true (about himself). This is deceit and treachery. “The believer is the mirror to the believer,” he sees in him his own picture and what needs perfecting and reformation.”