Hadith: A group of pagans went to excess in killing and committing illegal sexual intercourse

عن ابن عباس رضي الله عنهما: أن ناسًا من أهل الشرك كانوا قد قتلوا وأكثروا، وزنوا وأكثروا، فأتوا محمدًا صلى الله عليه وسلم فقالوا: إن الذي تقول وتدعو إليه لحَسَن، لو تخبرنا أن لما عملنا كفارة. فنزل: {والذين لا يدعون مع الله إلهًا آخر ولا يقتلون النفس التي حرم الله إلا بالحق ولا يزنون} [الفرقان: 68] ونزلت {قل يا عبادي الذين أسرفوا على أنفسهم لا تقنطوا من رحمة الله} [الزمر: 53].
[صحيح] - [متفق عليه]

Ibn ‘Abbās (may Allah be pleased with him) reported: A group of pagans, who went to excess in killing and committing illegal sexual intercourse, came to Muhammad (may Allah’s peace and blessings be upon him) and said: "What you say and invite people to is indeed good. If only you could tell us that there is expiation for what we did. Thereupon, it was revealed: {And those who do not invoke besides Allah another god, and who do not kill a soul whom Allah has forbidden, except in the course of justice, and who do not commit adultery} [Surat al-Furqān: 68]. And it was revealed: {Say [Allah says]: "O My slaves who have transgressed against themselves, do not despair of Allah’s mercy."} [Surat az-Zumar: 53]
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Ibn ‘Abbās (may Allah be pleased with him) informs in the Hadīth that a group of polytheists had excessively committed murders and adultery, but at the advent of Islam, they came to the Prophet (may Allah’s peace and blessings be upon him) and expressed their approval of what he invited people to and wished if he could inform them there was expiation for the major sins they had committed. So, these two verses were revealed. This shows Allah's mercy and forgiveness towards His servants and how He facilitates things for them. He accepted their repentance despite the plenty and seriousness of their sins. Were it not for that, they would have continued to their disbelief and transgression and would not have embraced this religion. We also derive from this Hadīth that entering Islam expiates people's past sins, provided they die as Muslims.