Sahih al-Bukhari 4591
Narrated Ibn `Abbas:
Regarding the Verse: "And say not to anyone who offers you peace (by accepting Islam), You are not 
a believer." There was a man amidst his sheep. The Muslims pursued him, and he said (to them) 
"Peace be on you." But they killed him and took over his sheep. Thereupon Allah revealed in that 
concern, the above Verse up to:-- "...seeking the perishable good of this life." (4.94) i.e. those sheep.
حَدَّثَنِي عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرٍو، عَنْ عَطَاءٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ ‏{‏وَلاَ تَقُولُوا لِمَنْ أَلْقَى إِلَيْكُمُ السَّلاَمَ لَسْتَ مُؤْمِنًا‏}‏‏.‏ قَالَ قَالَ ابْنُ عَبَّاسٍ كَانَ رَجُلٌ فِي غُنَيْمَةٍ لَهُ فَلَحِقَهُ الْمُسْلِمُونَ فَقَالَ السَّلاَمُ عَلَيْكُمْ، فَقَتَلُوهُ وَأَخَذُوا غُنَيْمَتَهُ، فَأَنْزَلَ اللَّهُ فِي ذَلِكَ إِلَى قَوْلِهِ ‏{‏عَرَضَ الْحَيَاةِ الدُّنْيَا‏}‏ تِلْكَ الْغُنَيْمَةُ‏.‏ قَالَ قَرَأَ ابْنُ عَبَّاسٍ السَّلاَمَ‏.‏