A man from the Ansar renounced Islam and joined the idolaters. Then Allah Most High revealed, "How shall Allah guide a people who disbelieved after their faith" [Al Imran 3:86], to the end of the verse. His people sent word of it to him, so he returned in repentance, and the Prophet ﷺ accepted that from him and let him go free.
أن رجلا من الأنصار ارتد عن الإسلام، ولحق بالمشركين، فأنزل الله تعالى: {كيف يهدي الله قوما كفروا بعد إيمنهم} إلى آخر الآية، فبعث بها قومه، فرجع تائبا، فقبل النبي ﷺ ذلك منه، وخلى عنه.
Narrated by Nasai (4068) and Ahmad (2218), and graded sahih by Ibn Hibban (4477) and Hakim (2/142), all from the hadith of Dawud bin Abu Hind, from Ikrima, from Ibn Abbas, who mentioned it. Its chain of transmission is sahih.In some of the narrations, it is said that the man then regretted what he had done and sent word to his people, "Ask the Messenger of Allah ﷺ whether there is any repentance for me." So his people came to the Messenger of Allah ﷺ and said, "So-and-so has regretted what he did, and he has ordered us to ask you whether there is any repentance." Then this verse was revealed,"How would Allah guide a people who disbelieved after their belief and had witnessed that the Messenger is true and had come to them with clear proofs. And Allah does not guide the wrongdoing people. Those, their recompense is that upon them is the curse of Allah and the angels and the people, all together, abiding therein eternally. The punishment will not be lightened for them, nor will they be reprieved, except for those who repent after that and reform themselves, for indeed Allah is Forgiving and Merciful" [Al Imran 3:86–89]He said, a man from his people carried this verse to him and read it to him. Harith then said, "By Allah, I know you to be truthful, and the Messenger of Allah ﷺ is more truthful than you, and Allah is the most truthful of the three." He said, so Harith returned, and became Muslim, and his Islam was good.This was narrated by Abdul-Razzaq in his Tafsir (1/131), except that it has not been established in the sahih reports that his name was Harith bin Suwayd, because this Harith was of Makhzum, not of the Ansar, whereas the well-known account that appears in the sahih reports is that he was an Ansari. Ibn Hajar likewise stated this in Al-Isabah.