Zayd bin Wahb narrates: Another man and I went to Sayyiduna Abdullah (may Allah be pleased with him), and he was standing in prayer, with two men on either side of him. When he concluded with the salam, the two of them asked him about a verse. He asked one of them: "Who taught you to recite it?" He said: "Umar (may Allah be pleased with him)." Then he asked the other: "Who taught you to recite it?" He said: "Abu Hakim" (or he said: "Abu Amrah"). He said: "Recite it as Umar (may Allah be pleased with him) taught you." Then he wept so much that his tears wetted the pebbles, and then he said: "Indeed, Sayyiduna Umar (may Allah be pleased with him) was a strong fortress for Islam; people would enter Islam and not leave it. Then when Sayyiduna Umar (may Allah be pleased with him) was martyred, that fortress was breached."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8769