Bushayr bin Yasar (may Allah have mercy on him) mentioned the hadith, and in it is that the Messenger of Allah (peace and blessings be upon him) said: "Will you take fifty oaths and become entitled to the killer of your companion?" They said: O Messenger of Allah! How can we testify when we were not present? He (peace and blessings be upon him) said: "The Jews will take fifty oaths and be cleared."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20735