Shaqiq narrates: When Umar (may Allah be pleased with him) was martyred, Abdullah bin Mas'ud (may Allah be pleased with him) travelled seven days to come to us and addressed us. He said: "The Commander of the Believers, Umar (may Allah be pleased with him), was struck by Abu Lu'lu'ah, the slave of al-Mughirah bin Shu'bah, while he was in the Fajr prayer, and he killed him." Saying this, he wept, and the people wept too. Then he said: "We, the Companions of Muhammad (peace and blessings be upon him), gathered and appointed as leader the best and most upright of us."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8803