Sahih Muslim Book - 54, Hadith - 7548

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى شَيْبَةَ وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ وَأَحْمَدُ بْنُ عَبْدَةَ الضَّبِّىُّ - وَاللَّفْظُ لاِبْنِ أَبِى شَيْبَةَ - قَالَ حَدَّثَنَا وَقَالَ الآخَرَانِ أَخْبَرَنَا سُفْيَانُ عَنْ عَمْرٍو عَنْ عَطَاءٍ عَنِ ابْنِ عَبَّاسٍ قَالَ لَقِىَ نَاسٌ مِنَ الْمُسْلِمِينَ رَجُلاً فِى غُنَيْمَةٍ لَهُ فَقَالَ السَّلاَمُ عَلَيْكُمْ. فَأَخَذُوهُ فَقَتَلُوهُ وَأَخَذُوا تِلْكَ الْغُنَيْمَةَ فَنَزَلَتْ (وَلاَ تَقُولُوا لِمَنْ أَلْقَى إِلَيْكُمُ السَّلَمَ لَسْتَ مُؤْمِنًا) وَقَرَأَهَا ابْنُ عَبَّاسٍ السَّلاَمَ.

It was narrated that Ibn 'Abbas, may Allah be pleased with them, said: "Some Muslim people met a man with his small flock of sheep and he said: 'As-salamu 'alaikum (peace be upon you).' They took him and killed him, then they took that small flock of sheep. Then the Verse: "...And say not to anyone who greets you (by embracing Islam): "You are not a believer..." was revealed."