Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "I think that on the day Umar (may Allah be pleased with him) died, nine-tenths of knowledge was taken up with him. And I think that if the knowledge of Umar (may Allah be pleased with him) were placed in one pan of the scale, and the knowledge of those who came after him in the other pan, the knowledge of Umar (may Allah be pleased with him) would outweigh it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8778