Musannaf Ibn Abi Shayba 27:369

Abu Mu'awiyah narrated to us, from al-A'mash, from Zayd ibn Wahb, who said: Two men came to 'Abdullah. One of them said, "O Abu 'Abd al-Rahman, how do you recite this verse?" 'Abdullah said to him, "Who taught you to recite?" He said, "Abu Hakim al-Muzani." He said to the other, "Who taught you to recite?" He said, "Umar taught me to recite." He said, "Recite as Umar taught you to recite." Then he wept until his tears fell onto the pebbles. Then he said, "Indeed, Umar was a fortified fortress for Islam; one would enter it and not exit from it. When Umar died, the fortress was breached, and now one exits from it and does not enter it."