Hadith: What you say and invite people to is indeed good. If only you could tell us whether there is an expiation for what we did

عَنِ ‌ابْنِ عَبَّاسٍ رَضِيَ اللهُ عَنْهُمَا:
أَنَّ نَاسًا مِنْ أَهْلِ الشِّرْكِ، كَانُوا قَدْ قَتَلُوا وَأَكْثَرُوا، وَزَنَوْا وَأَكْثَرُوا، فَأَتَوْا مُحَمَّدًا صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فَقَالُوا: إِنَّ الَّذِي تَقُولُ وَتَدْعُو إِلَيْهِ لَحَسَنٌ، لَوْ تُخْبِرُنَا أَنَّ لِمَا عَمِلْنَا كَفَّارَةً، فَنَزَلَ {وَالَّذِينَ لا يَدْعُونَ مَعَ اللهِ إِلَهًا آخَرَ وَلا يَقْتُلُونَ النَّفْسَ الَّتِي حَرَّمَ اللهُ إِلا بِالْحَقِّ وَلا يَزْنُونَ}[الفرقان: 68]، وَنَزَلَت: {قُلْ يَا عِبَادِيَ الَّذِينَ أَسْرَفُوا عَلَى أَنْفُسِهِمْ لا تَقْنَطُوا مِنْ رَحْمَةِ اللهِ} [الزمر: 53].
[صحيح] - [متفق عليه] - [صحيح البخاري: 4810]

Ibn ‘Abbās (may Allah be pleased with him) reported:
A group of polytheists, 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 whether there is an 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 the following verse 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

Some men from the polytheists came to the Prophet (may Allah's peace and blessings be upon him), as they had gone to extremes in killing and committing adultery, and said: Indeed, what you call to (Islam) and its teachings are something good. But, as for our condition and the polytheism and major sins we have committed, is there an expiation for them?
Thereupon, the two verses were revealed, as Allah accepted the repentance of those people despite their numerous and severe sins. Had it not been for that, they would have persisted in their disbelief and transgression and would not have embraced this religion.