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"O you who believe! Let not a group scoff at another group, it may be that the latter are better than the former; nor let (some) women scoff at other women, it may be that the latter are better than the former, nor defame one another, nor insult one another by nicknames, How bad is it, to insult one's brother after having Faith [i.e, to call your Muslim brother (a faithful believer) as: "O sinner", or "O wicked", etc.], And whosoever does not repent, then such are indeed Dhaalimoon (wrong-doers, etc.)." [Al-Hujuraat (49):11]

Narrated Abu Musa:

The Prophet said, "The example of guidance and knowledge with which Allah has sent me is like abundant rain falling on the earth, some of which was fertile soil that absorbed rain water and brought forth vegetation and grass in abundance. (And) another portion of it was hard and held the rain water and Allah benefited the people with it and they utilized it for drinking, making their animals drink from it and for irrigation of the land for cultivation. (And) a portion of it was barren which could neither hold the water nor bring forth vegetation (then that land gave no benefits). The first is the example of the person who comprehends Allah's religion and gets benefit (from the knowledge) which Allah has revealed through me (the Prophets and learns and then teaches others. The last example is that of a person who does not care for it and does not take Allah's guidance revealed through me (He is like that barren land.)" [The Book of Knowledge Volume 1, Book 3, Hadeeth 79]