Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "The people of two different religions shall not inherit from one another, nor can the follower of one religion testify against another. Only the Ummah of Muhammad has this right, that its testimony is valid even against others."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20616