Quran 3:135 from Surah Aal-i-Imraan on Divine Marvels.
وَٱلَّذِينَ إِذَا فَعَلُواْ فَٰحِشَةً أَوۡ ظَلَمُوٓاْ أَنفُسَهُمۡ ذَكَرُواْ ٱللَّهَ فَٱسۡتَغۡفَرُواْ لِذُنُوبِهِمۡ وَمَن يَغۡفِرُ ٱلذُّنُوبَ إِلَّا ٱللَّهُ وَلَمۡ يُصِرُّواْ عَلَىٰ مَا فَعَلُواْ وَهُمۡ يَعۡلَمُونَ
English translation: And those when they did immorality or wronged themselves they remember Allah then ask forgiveness for their sins and who (can) forgive the sins except Allah And not they persist on what they did while they know
Transliteration: wa-alladhina idha fa'alu fahishatan aw zalamu anfusahum dhakaru al-laha fa-is'taghfaru lidhunubihim waman yaghfiru al-dhunuba illa al-lahu walam yusirru ala ma fa'alu wahum ya'lamuna