Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "If the knowledge of Umar (may Allah be pleased with him) were placed in one pan of a scale, and all the knowledge of the people of the earth were placed in the other pan, his knowledge would outweigh all of their knowledge." Waki' said: Al-A'mash said: I found this strange, so I went to Ibrahim and mentioned it to him. He said: "What did you find strange about it? By Allah, Sayyiduna Abdullah (may Allah be pleased with him) said something even greater than that. He said: 'I think that nine-tenths of knowledge departed on the day Umar (may Allah be pleased with him) departed.'"
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8777