Musnad Ahmad 32:2046

'Affan narrated to us, Wuhaib narrated to us, Khalid al-Hadhdha' narrated to us, from Abu Qilabah, from Anas ibn Malik, from the Prophet, peace and blessings of Allah be upon him, who said: "The most merciful of my nation to my nation is Abu Bakr, and the most severe of them regarding the religion of Allah is 'Umar" — and 'Affan said once: "regarding the command of Allah is 'Umar" — "and the most truthful of them in modesty is 'Uthman, and the most knowledgeable of them in inheritance laws is Zayd ibn Thabit, and the best reciter of them of the Book of Allah is Ubayy ibn Ka'b, and the most knowledgeable of them in the lawful and the unlawful is Mu'adh ibn Jabal. Verily, every nation has a trustee, and the trustee of this nation is Abu 'Ubaydah ibn al-Jarrah."