It is narrated from Sayyiduna Abdullah bin Salam (may Allah be pleased with him) that when the people were stirred up over the matter of Sayyiduna Uthman (may Allah be pleased with him), he said: "O people! Do not kill this elderly man; rather, seek redress from him (for your grievances). For whenever a nation killed its prophet and (then) its affairs were set right, they were not set right without the blood of seventy thousand people being shed; and whenever a nation killed its caliph and its affairs were set right, they were not set right without the blood of forty thousand people being shed." But the people paid no heed to what he said, and they killed him (Sayyiduna Uthman, may Allah be pleased with him). Then he (Abdullah bin Salam) sat waiting for Sayyiduna Ali (may Allah be pleased with him) on the road and asked (him): "Where do you intend to go?" Sayyiduna Ali (may Allah be pleased with him) said: "I intend to go to Iraq." He said: "Do not go to Iraq; hold fast to the pulpit of the Messenger of Allah (peace and blessings be upon him)." (Hearing this,) some of the companions of Sayyiduna Ali pounced on him and intended to harm him, whereupon Sayyiduna Ali (may Allah be pleased with him) said: "Leave him, for he is one of us, the People of the House." Then, when Sayyiduna Ali (may Allah be pleased with him) was martyred, Ibn Salam said to Ibn Ma'qil: "This is the head of the forty (years), and at its head there will be a peace settlement. And whenever a nation killed its prophet, seventy thousand people were killed in retribution for him; and whenever a nation killed its caliph, forty thousand people were killed in retribution for him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13963