Narrated Ibn 'Abbas: The Muslims met a man with some sheep of his. He said: Peace be upon you. But they killed him and took those few sheep. Thereupon the following Qur'anic verse was revealed: "...And say to anyone who offers you a salutation: Thou art none of believer, coveting the perishable good of this life." meaning these few sheep.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ عِيسَى، حَدَّثَنَا سُفْيَانُ، حَدَّثَنَا عَمْرُو بْنُ دِينَارٍ، عَنْ عَطَاءٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ لَحِقَ الْمُسْلِمُونَ رَجُلاً فِي غُنَيْمَةٍ لَهُ فَقَالَ السَّلاَمُ عَلَيْكُمْ فَقَتَلُوهُ وَأَخَذُوا تِلْكَ الْغُنَيْمَةَ فَنَزَلَتْ { وَلاَ تَقُولُوا لِمَنْ أَلْقَى إِلَيْكُمُ السَّلاَمَ لَسْتَ مُؤْمِنًا تَبْتَغُونَ عَرَضَ الْحَيَاةِ الدُّنْيَا } تِلْكَ الْغُنَيْمَةَ .
sunnah.com