O ye who believe! avoid suspicion as much (as possible): for suspicion in some cases is a sin: and spy not on each other nor speak ill of each other behind their backs. Would any of you like to eat the flesh of his dead brother? Nay ye would abhor it...but fear Allah: for Allah is Oft-Returning Most Merciful.
[surah Hujurat; 49:12]
Allah’s Apostle (salAllahu alayhi wasalam) said: Do you know what is backbiting? They (the Companions) said: Allah and His Apostle (salAllahu alayhi wasalam) know best. Thereupon he (the Prophet) said: Backbiting implies your talking about your brother in a manner which he does not like. It was said to him: What is your opinion about this that if I actually find (that failing) in my brother which I made a mention of? He said: If (that failing) is actually found (in him) what you assert, you in fact backbited him, and if that is not in him it is a slander.
[Sahih Muslim, Hadith 6265]