Musnad Ahmad 2:152

Muhammad ibn Fudayl narrated to us, Isma'il ibn Sami' narrated to us, from Muslim al-Batin, from Abu al-Bakhtari, who said: 'Umar said to Abu 'Ubaydah ibn al-Jarrah: "Stretch out your hand so that I may pledge allegiance to you, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'You are the trustworthy one of this nation.'" Abu 'Ubaydah said: "I would not step forward in front of a man whom the Messenger of Allah, peace and blessings of Allah be upon him, commanded to lead us in prayer, so he led us in prayer until he died."