The Believer is a Mirror for His Brother
From Abu Hurairah (radiyallaahu 'anhu) who said that Allaah's Messenger (:arabic5:) 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. Say 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.
* Reported by al-Bukharee in al-Adabul Mufrad (no. 239), Aboo Dawood (Eng. Trans. 3/1370/no.4900) and others. Its chain of narration is hasan.