Our Lord, We Have Wronged Ourselves
رَبَّنَا ظَلَمْنَا أَنْفُسَنَا وَإِنْ لَمْ تَغْفِرْ لَنَا وَتَرْحَمْنَا لَنَكُونَنَّ مِنَ الْخَاسِرِينَ
Rabbana zalamna anfusana wa in lam taghfir lana wa tarhamna la-nakunanna minal-khasirin.
Our Lord, we have wronged ourselves, and if You do not forgive us and have mercy on us we will surely be among the losers.
What it means
These are the words Adam and Hawwa said after their mistake, and the Quran contrasts them with the arrogance of Iblis, who argued instead of admitting fault. Sin itself did not separate Adam from Allah; refusing to own it is what ruins a person. The phrase we have wronged ourselves is exact, because disobedience harms no one else first.
When and how
Say it when you have done something you regret, before making excuses for yourself.
Its virtue
The Quran records that Adam received words from his Lord and turned to Him, and that Allah accepted his repentance and guided him.
Source
Quran 7:23
Quranic text from Tanzil.net, Creative Commons Attribution 3.0.