Rafi' bin Khadij (may Allah be pleased with him) said: A man of the Ansar was found killed in the morning at Khaybar. His heirs mentioned this to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) asked: "Do you have two witnesses who will testify to this killing?" They said: O Messenger of Allah (peace and blessings be upon him)! No Muslim was present there; there are only Jews there, and they are capable of doing even greater than this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20525