Habib Abdullah bin Husayn bin Tahir (may Allah have mercy upon him and benefit us by him) derived four different meanings from the hadith of the Messenger of Allah (may Allah bless him and grant him peace):
المُؤْمِنُ مِرَآةُ المُؤْمِنِ
“One believer is a mirror to another.”
One meaning, he said, is that the believer sees good qualities in his brother so he attempts to emulate them, or he sees bad qualities in his brother so he knows that he possesses similar qualities and attempts to eradicate them.
A second meaning is that the believer sees a fault in his brother so he tells him to remove it. He is thus like a mirror to his brother.
A third meaning is that the believer appears to his brother according to his brother’s inward state. If his heart is pure then he sees his brothers as being pure of heart like himself, and he holds a good opinion of them. If, however, his heart is sick he sees his brothers as being like him and he holds a bad opinion of them. The believer thus projects what is in his heart onto his brothers. Thus if someone sees the Prophet ﷺ in a beautiful form in a dream then this shows that the person is in a good state. If, however he does not see the Prophet ﷺ in a beautiful form then this shows that the person has some kind of deficiency.
A further meaning is that if a believer’s faith is complete, Allah manifests Himself to his heart. One of Allah’s names is ‘al-Mu’min’, meaning ‘the Faithful.’ Thus the meaning of the hadith would be: ‘the believer, or more specifically the heart of the believer is the place, or the mirror, to which Allah ‘the Faithful’ manifests himself.
 Narrated by Abu Daud