In the Name of Allāh, the Most Merciful, may His Ṣalāh and Salām be upon His last Messenger to all of mankind, to proceed.1
ʿAbd al-Raḥmān ibn Mahdī (raḥimahullāh), the Imām, the Ḥujjah, said:
If I did not hate that Allāh be disobeyed, I would have wished that there was no one left in the whole city except that he has backbitten me! What is more rewarding than a man finding a good deed (written) in his book for something he did not even do?!2
May Allāh grant us something of the sincerity and patience that our Salaf exemplified!
“If I did not hate that Allāh be disobeyed…”
Meaning that backbiting is ḥarām, and to wish that the Muslims would backbite would be wishing that Allāh be disobeyed. However, to stress the benefit of being backbitten, he goes on to say:
“I would have wished that there was no one left in the whole city except that he has backbitten me!”
When someone is backbitten and slandered, naturally he feels disturbed and upset, and even a sense of loss. So people need someone to point out that there is a great amount of good in being the focus of people’s backbiting. This advice of Abdur-Raḥmān Mahdī seems to be from the baab of mutually advising each other to have patience, as mentioned in Sūrah al-ʿAṣr.
Apparently, this was in response to him hearing that some people were talking about him, or possibly someone was complaining to him about having been backbitten, and Allāh knows best. He says that if it was permissible to wish for others to fall into disobedience, he would have wished that they all would backbite him, and then he clarifies the reason for saying this:
“What is more rewarding than a man finding a good deed (written) in his book for something he did not even do?!”
Since the one who backbites someone (in other than the permissible, sometimes obligatory, forms of backbiting) forwards to that person his good deeds or will be made to be accountable for the bad deeds of his victim, as mentioned in the ḥadīth of Al-Bukhārī:
“The bankrupt one from my nation will come on the Day of Judgement with deeds of prayer, fasting, and charity, but he has insulted this one, maligned that one, taken the money of this one, spilled the blood of that one, or hit another one. So some of his good deeds will be given to this one and that one. If no good deeds remain, then it will be judged that their bad deeds be taken and cast upon him, then he will be cast into the Fire.”
And we ask Allāh for protection.
So then his statement:
“a good deed (written) in his book for something he did not even do”
refers to what is mentioned in the ḥadīth that the one who is maligned will be given some of the good deeds of the one who maligned him.