Dua #1098•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.

For one you have insulted

Hisnul Muslim
#1098

قال صلى الله عليه وسلم:'اللَّهُم فأيما مؤمن سببته فاجعل ذلك له قربة إليك يوم القيامة'

Transliteration

Allāhumma fa'ayyumā mu'minin sababtuhufaj`al dhālika lahu qurbatan ilayka yawma ‘l-qiyāmah.

Translation

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