Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that two men came to him concerning the fitnah of Ibn al-Zubayr and said: You know what the people have done? You are the son of Sayyiduna Umar (may Allah be pleased with him) and a Companion of the Messenger of Allah (peace and blessings be upon him); what prevents you from coming out? He said: What prevents me is that Allah Almighty has made the blood of a Muslim brother sacred. They said: Has Allah Almighty not said: "And fight them until there is no fitnah and the religion, all of it, is for Allah" [Surah al-Anfal: 39]? He said: We fought until that was so; but now you fight so that there will be fitnah and the religion will be for other than Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16805