'Abdullaah ibn 'Umar rad iallaahu 'anhu relates that the Prophet sallallaahu `alayhi wa sallam mounted the pulpit and said in a loud voice: "O You who have accepted Islaam with your tongues, but in whose heart eemaan (faith) has not yet entered! Do not annoy the Muslims' nor revile them, nor seek out their faults. For whosoever seeks out the fault of his Muslim brother, Allaah will seek out his faults. He whose faults are sought out by Allaah, then Allaah will expose him, even if he is inside his house." Related by Muslim.
'Ubaadah ibn as-Saamit radiallaahu `anhu related that the Prophet sallallaahu `alayhi wa sallam said: "Whosoever seeks forgiveness for the believing men and the believing women, Allaah will record for that person - equivalent to every believing man and believing woman -a good deed."
Hasan: Related by al-Haythamee in Majma'az-Zawaa'id (1/210) where he said: "The isnaad is jayyid."
Allah `azza wa jall says: “O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful.” [Al-Hujurat 49:12]
Allah `azza wa jall ordered avoiding much of the suspicion because some of it is sin, and He followed this order with the prohibition of spying, pointing out that spying in most cases does not occur except due to bad assumptions.
The way of the Muslim – the general rule - is hiding others' faults and having a good opinion of others. This is why Allah `azza wa jall ordered believers to have a good opinion of people when they hear slanders of their Muslim brothers.
Allah `azza wa jall clarified the true position which every Muslim must understand. He said subhanahu wa ta`ala:
“Why, when you heard it, did not the believing men and believing women think good of themselves [i.e. one another] and say, 'This is an obvious falsehood'?” [An-Noor 24:12]” [Hisham Ismail As-Sini, Manhaj Ahl As-Sunnah wal Jama`ah fin Naqd wal Hukm `ala al Akhirin, Al Muntada, London, 1992, p. 21]
Dr. Mustafa as-Siba'i said, “Because having a good opinion and then regretting is better than having a bad opinion and then regretting.” [as-Sibai, Hakadha `allamatni al Hayat, al Maktab al Islami, Beirut, 1984, vol. 1, p. 42]
Taken from: www.islaam.com