Masruq (may Allah have mercy on him) narrates: "I compared the Companions of Muhammad (peace and blessings be upon him) and found that their knowledge ended with six: Umar, Ali, Abdullah bin Mas'ud, Abu al-Darda and Zayd bin Thabit (may Allah be pleased with them). Then I compared those six and found that their knowledge ended with Ali and Abdullah (may Allah be pleased with them both)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8481