Zayd bin Wahb narrates: We came to Sayyiduna Ibn Mas'ud (may Allah be pleased with him) and found him praying, so we waited until he finished his prayer. Then two men who had disagreed about a verse came to him. One of them recited it, and Sayyiduna Abdullah (may Allah be pleased with him) said: "You recited well. Who taught you to recite it?" He said: "Abu Hakim al-Muzani taught me." Then he had the other recite and said: "Who taught you to recite it?" He said: "Umar bin al-Khattab (may Allah be pleased with him) taught me." Sayyiduna Abdullah (may Allah be pleased with him) wept until his tears coloured the pebbles. Then he said: "Recite it as Umar (may Allah be pleased with him) taught you." Then he drew a circle with his hand and said: "Indeed, Umar (may Allah be pleased with him) was a strong fortress for Islam; people would enter it and not leave it. Then when Umar (may Allah be pleased with him) died, the fortress was breached, and now people leave it and do not enter it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8773