Sayyiduna Rafi' bin Khadij (may Allah be pleased with him) said: A man of the Ansar was found killed in Khaybar. His guardians came to the Prophet (peace and blessings be upon him) and related the whole incident. The Prophet (peace and blessings be upon him) said: "Do you have two witnesses who will testify to the killing of your companion?" They said: "O Messenger of Allah, there was no Muslim there; they are only Jews, and they dare to commit even greater sins than this." And he mentioned the complete hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16492