It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever walks with a people making it appear that he is a witness when he is not a witness is a false witness; whoever helps in a dispute without knowledge remains under the displeasure of Allah until he desists; and killing a Muslim is disbelief, and abusing him is sinfulness."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوكالة / 11444