It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Indeed, Allah, the Mighty and Majestic, has not made lawful in the fitnah (civil strife) anything that He made unlawful before it. What is the matter with one of you, that he comes to his brother and greets him with peace, then after that comes and kills him?"
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7746