Hadith: A spy from among the polytheists came to the Prophet (may Allah's peace and blessings be upon him) while he was on a journey.

عن سلمة بن الأكوع رضي الله عنه قال: «أَتَى النَّبِيَّ صلى الله عليه وسلم عَيْنٌ مِنْ الْمُشْرِكِينَ، وَهُوَ فِي سَفَرِهِ، فَجَلَسَ عِنْدَ أَصْحَابِهِ يَتَحَدَّثُ، ثُمَّ انْفَتَلَ، فَقَالَ النَّبِيُّ صلى الله عليه وسلم : اُطْلُبُوهُ وَاقْتُلُوهُ فَقَتَلْتُهُ، فَنَفَّلَنِي سَلَبَهُ». فِي رِوَايَةٍ «فَقَالَ: مَنْ قَتَلَ الرَّجُلَ؟ فَقَالُوا: ابْنُ الأَكْوَعِ فَقَالَ: لَهُ سَلَبُهُ أَجْمَعُ».
[صحيح] - [الرواية الأولى: متفق عليها. الرواية الثانية رواها مسلم]

Salamah ibn al-Akwa‘ (may Allah be pleased with him) said: "A spy from among the polytheists came to the Prophet (may Allah's peace and blessings be upon him) while he was on a journey. The spy stayed with the Prophet's Companions (may Allah be pleased with them) and talked to them, and then he left. Thereupon, the Prophet (may Allah's peace and blessings be upon him) said: 'Go in pursuit of him and kill him.' I killed him, and the Prophet (may Allah's peace and blessings be upon him) gave me his belongings (as an extra share)." According to another Hadīth, the Prophet (may Allah's peace and blessings be upon him) said: "Who killed the spy?" They (the Companions) said: "Ibn al-Akwa‘." He said: "All the belongings of the killed man are for him (Salamah)."
Sahih/Authentic. - [Muslim]

Explanation

This Hadīth highlights the Islamic ruling on a person who spies over the Muslims while he is one of the combatant disbelievers. Salamah ibn Al-Akwa‘ (may Allah be pleased with him) said: "A spy from among the polytheists came to the Messenger of Allah (may Allah's peace and blessings be upon him)." The Arabic version of the Hadīth uses the word "‘ayn" (eye) in reference to the spy because the act of spying depends upon the eye, and because a spy is mainly concerned with and focused on watching, as if his whole body has become an eye. "The spy stayed with the Prophet's Companions (may Allah be pleased with them) and talked with them and then went away. Thereupon, the Prophet (may Allah's peace and blessings be upon him) said: 'Pursue and kill him.' So, I killed him." Here, Salamah says that he pursued the spy, caught him, and killed him. "And the Prophet (may Allah's peace and blessings be upon him) gave me his belongings (as an extra share)." The Prophet (may Allah's peace and blessings be upon him) gave Salamah all the belongings of the man, including clothes, weapons, riding mount, and its saddle, along with all the money, gold, and silver which the killed person had.