Sunan an-Nasa'i 4068
It was narrated that Ibn 'Abbas said:
"A man from among the Ansar accepted Islam, then he apostatized and went back to Shirk. Then he regretted that, and sent word to his people (saying): 'Ask the Messenger of Allah [SAW], is there any repentance for me?' His people came to the Messenger of Allah [SAW] and said: 'So and so regrets (what he did), and he has told us to ask you if there is any repentance for him?' Then the Verses: 'How shall Allah guide a people who disbelieved after their Belief up to His saying: Verily, Allah is Oft-Forgiving, Most Merciful' was revealed. So he sent word to him, and he accepted Islam."
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ بَزِيعٍ، قَالَ حَدَّثَنَا يَزِيدُ، - وَهُوَ ابْنُ زُرَيْعٍ - قَالَ أَنْبَأَنَا دَاوُدُ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ كَانَ رَجُلٌ مِنَ الأَنْصَارِ أَسْلَمَ ثُمَّ ارْتَدَّ وَلَحِقَ بِالشِّرْكِ ثُمَّ تَنَدَّمَ فَأَرْسَلَ إِلَى قَوْمِهِ سَلُوا لِي رَسُولَ اللَّهِ صلى الله عليه وسلم هَلْ لِي مِنْ تَوْبَةٍ فَجَاءَ قَوْمُهُ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالُوا إِنَّ فُلاَنًا قَدْ نَدِمَ وَإِنَّهُ أَمَرَنَا أَنْ نَسْأَلَكَ هَلْ لَهُ مِنْ تَوْبَةٍ فَنَزَلَتْ ‏{‏ كَيْفَ يَهْدِي اللَّهُ قَوْمًا كَفَرُوا بَعْدَ إِيمَانِهِمْ ‏}‏ إِلَى قَوْلِهِ ‏{‏ غَفُورٌ رَحِيمٌ ‏}‏ ‏"‏ ‏.‏ فَأَرْسَلَ إِلَيْهِ فَأَسْلَمَ ‏.‏