Abdur-Rahman bin al-Harith bin Hisham says: I heard a bishop from the people of Najran speaking to Sayyiduna Umar (may Allah be pleased with him), saying: O Commander of the Believers, beware of the killer of three. Sayyiduna Umar (may Allah be pleased with him) said: What is meant by the killer of three? He said: The man who comes to the ruler (imam) with a lie, and the ruler kills the man about whom he spoke; so it is as though he has killed himself, that man, and the ruler.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16677