Alqamah bin Qays (may Allah have mercy on him) says: A man came to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and said: "A man committed zina with a woman, then both repented and reformed themselves; can the two of them marry?" He recited this verse: "Then indeed your Lord, for those who did evil in ignorance and then repented after that and reformed, indeed your Lord, after that, is Forgiving." [Surah al-Nahl: 119] He recited the verse to him again and again until he thought that he had granted a concession in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13885