From Abu Hurairah (radiyallaahu 'anhu) who said that Allaah's Messenger (salallaahu 'alaihi wa'sallam) said:
The believer is a mirror for the believer, and the believer is the brother of the believer. He safeguards his property for him and defends him from behind.
So the description of his being a 'mirror' is very precise and profound showing the culmination of brotherhood and solidarity. So your brother, O servant of Allaah, is an image of you yourself. Sao if he behaves badly it is as if you are the one who has behaved badly, and if he makes a mistake, it is as if you have made a mistake. So he is a mirror for you and then an image of you yourself! So do not treat him except with mildness and gentleness. If you do not behave with your brother in this manner, then this will be something which weakens: