Reference : Hisn al-Muslim 230

قال صلى الله عليه وسلم: "اللَّهُم فأيما مؤمن سببته فاجعل ذلك له قربة إليك يوم القيامة"
Allāhumma fa'ayyumā mu'minin sababtuhu 
faj`al dhālika lahu qurbatan ilayka yawma ‘l-qiyāmah.


O Allah, whomever of the believers I have abused, give him the reward of a sacrificial slaughter for it on the Day of Resurrection.

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/171, Muslim 4/2007. The wording 
in Muslim's report is: 'make it a purification for him and a source of 
mercy.

Chapter: For one you have insulted