Ibn Salamah bin al-Akwa' narrates from his father (Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him)) that during a journey a spy of the polytheists came to the Prophet (peace and blessings be upon him). He sat with the Companions (may Allah be pleased with them), talking, and then slipped away. He (peace and blessings be upon him) said: "Catch him and kill him." The narrator says: I killed him and took his belongings.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18436