Amir ibn al-Adbat al-Ashja'i was a secret Muslim from a tribe that had not accepted Islam, killed before the Conquest of Makkah by Muhallim ibn Juthama, who ignored his greeting of salam out of an old grudge and took his belongings. Allah revealed [4:94], forbidding the believers to tell those who offer peace that they are not believers for worldly gain. The Prophet ﷺ settled the dispute after Hunayn by paying his blood money himself.
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يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا ضَرَبْتُمْ فِي سَبِيلِ اللَّهِ فَتَبَيَّنُوا وَلَا تَقُولُوا لِمَنْ أَلْقَىٰ إِلَيْكُمُ السَّلَامَ لَسْتَ مُؤْمِنًا تَبْتَغُونَ عَرَضَ الْحَيَاةِ الدُّنْيَا فَعِندَ اللَّهِ مَغَانِمُ كَثِيرَةٌ ۚ كَذَٰلِكَ كُنتُم مِّن قَبْلُ فَمَنَّ اللَّهُ عَلَيْكُمْ فَتَبَيَّنُوا ۚ إِنَّ اللَّهَ كَانَ بِمَا تَعْمَلُونَ خَبِيرًا
“O believers! When you struggle in the cause of Allah, be sure of who you fight. And do not say to those who offer you [greetings of] peace, "You are no believer!" -- seeking a fleeting worldly gain. Instead, Allah has infinite bounties [in store]. You were initially like them, then Allah blessed you [with Islam]. So be sure! Indeed, Allah is All-Aware of what you do.”
This is the well-known asbab al-nuzul: revealed rebuking Muhallim ibn Juthama's sariyya for killing Amir ibn al-Adbat after he had greeted them with the Islamic salam, described in his own bio.