Abu Ubaidah ibn al-Jarrah
His rule was one of the few moments in the history of Islam where Muslims were united as a single community. Abdullah ibn Masʿud would often weep whenever the subject of Omar was brought up. He said: "Umar was a fortress of Islam. People would enter Islam and not leave. When he died, the fortress was breached and now people are going out of Islam". Abu Ubaidah ibn al-Jarrah before Omar died famously said: "If Omar dies, Islam would be weakened". People asked him why and his reply was "You will see what I am speaking about if you survive."