Bismillaah Al-Hamdulillaah wa salatu wa salaamu 'ala rasulullaah Amma ba'd
Invoking Allaah to Reward Someone You have Spoken ill to
Allaahumma fa Ayyummaa mu`minin sababtuhu faj'al dhaalika lahu Qurbatan ilaika Yawmal-Qiyaamati (Oh Allaah, whomever of the believers I have abused, give him the reward of a sacrificial slaughter for it on the Day of Resurrection.)
(Al-Bukharee; Fat-H al-Baari 11/171; Muslim 4/2007 - the wording in Muslim is: 'make it a purification for him and a source of mercy.')
Source: Hisnul Muslim (Fortress of the Muslim)
Subhanak Allaahuma wa bihamdika ash-hadu anlaa illaaha illa anta astaghfiruka wa atubu ilayk
If I said anything correct, then it is from Allaah (subhanahu wa taa'ala), and if I erred, then that is from me and shaytan.