And spy not, neither backbite one another. Would one of you love to eat the flesh of his dead brother? Ye abhor that (so abhor the other)! And keep your duty (to Allah). Lo! Allah is Relenting, Merciful. (49:12) And this interpretation may indicate that first a man is involved in suspicion, and suspicion leads him to spying, and spying too ends in revealing hidden faults of individuals and this is the source of slander.
2- Slander is not compatible with belief As it is stipulated in Barae tradition: خَطَبَنا رَسُولُ اللهِ (صلى الله علیه وآله) حَتَّى اسْتَمَعَ الْعَوائِقُ فِی بُیُوتِها فَقالَ: «یا مَعْشَرَ مَنْ آمَنَ بِلِسانِهِ وَ لَمْ یُؤْمِنْ بِقَلْبِهِ لا تَغْتابُوا الْمُسْلِمِینَ وَ لا تَتَّبِعُوا عَوْراتِهِمْ فَاِنَّهُ مَنْ تَتَبَّعَ عَوْرَةَ اَخِیهِ تَتَبَّعَ اللهُ عَوْرَتَهُ وَ مَنْ تَتَبَّعَ اللهُ عَوْرَتَهُ یَفْضَحُهُ وَ لَوْ فِی جَوْفِ بَیْتِهِ».
Barae says: Prophet (S) spoke for us, and so loudly that even the girls in the houses too heard. He said: O the group who has believed by tongue, but not by heart.
Do not slander about a Muslim, and do not spy their secret affairs, the one who is curious for the secret affairs of his religious brothers, God shall reveal his own secret affairs and scandalizes him inside his own house.[^2] The point of non-compatibility of slander with the spirit of belief may indicate that the first sign of belief is observing the right of a believer, and this is in contradiction with slander.
3- Slander is considered as spreading obscenity Imam As-Sadiq (A.S.) said: «مَنْ قالَ فِی مُؤْمِن ما رَأَتْهُ عَیْناهُ وَ سَمِعَتْهُ اُذُناهُ فَهُوَ مِنَ الَّذِینَ قالَ اللهُ عَزَّ وَ جَلَّ: ( اِنَّ الَّذِینَ یُحِبُّونَ اَنْ تَشِیعَ الْفاحِشَةُ فِی الَّذِینَ آمَنُوا لَهُمْ عَذابٌ اَلِیمٌ.» One who says what he has seen and heard about a believer, is among those that God has told about them: those who like the vices to spread among believers, there shall be a painful chastisement for them.[^3] The point of this issue is also clear, because considering that in most cases faults are included among religious faults, revealing hidden faults of people may cause the others to take impertinence and insolence for performing sin and disobedience, and consequently spreading obscenity.
4- The corruption of slander is more severe than adultery In the famous tradition of Jaber and Abou Saeid Khadri, it is narrated from Prophet (S): «اَلْغِیبَةُ اَشَدُّ مِنَ الزِّنا».