Mishkat al-Masabih 3961
Salama b. al-Akwa‘ said:
A spy of the polytheists who came to the Prophet when he was on a journey sat and talked with his companions and afterwards withdrew. The Prophet then said, “Look for him and kill him,” so I killed him and the Prophet gave me the spoil taken from him. 

(Bukhari and Muslim.)
وَعَنْ سَلَمَةَ بْنِ الْأَكْوَعِ قَالَ أَتَى النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَيْنٌ مِنَ الْمُشْرِكِينَ وَهُوَ فِي سَفَرٍ فَجَلَسَ عِنْدَ أَصْحَابِهِ يَتَحَدَّثُ ثُمَّ انْفَتَلَ فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «اطْلُبُوهُ وَاقْتُلُوهُ» . فَقَتَلْتُهُ فنفَّلَني سلبَه