Sunan Ibn Majah, Chapter: The Book of the Sunnah

that the Messenger of Allah (saw) said: "The most merciful of my Ummah towards my Ummah is Abu Bakr; the one who adheres most sternly to the religion of Allah is Umar; the most sincere of them in shyness and modesty is Uthman; the best judge is Ali bin Abu Talib; the best in reciting the Book of Allah is Ubayy bin Kab; the most knowledgeable of what is lawful and what is unlawful is Muadh bin Jabal; and the most knowledgeable of the rules of inheritance (Faraid) is Zaid bin Thabit. And every nation has a trustworthy guardian, and the trustworthy guardian of this Ummah is Abu Ubaidah bin Jarrah." (Sahih)