Musannaf Ibn Abi Shayba 33:226

Marwan ibn Mu'awiyah narrated to us, from Muhammad ibn Suqah, who said: I came to Nu'aym ibn Abi Hind, and he brought out a scroll to me. In it was: "From Abu Ubaydah ibn al-Jarrah and Mu'adh ibn Jabal to Umar ibn al-Khattab: Peace be upon you. To proceed: We knew you when the affair of your own self was important to you, and you have now become the one in charge of the affair of this nation, its red and its black. The noble and the lowly, the enemy and the friend sit before you, and each has their share of justice. So look at how you will be at that time, O Umar, for we warn you of a day when faces will be humbled, hearts will be agitated, and arguments will be severed. He possesses their subjugation by His might, and the creation are submissive to Him, hoping for His mercy and fearing His punishment. We used to say that the affair of this nation would return in its final time to being brothers in public and enemies in private. We seek refuge in Allah that our letter to you should descend to any place other than the place it descended from in our hearts, for we have written it as advice to you. And peace be upon you." Umar ibn al-Khattab wrote back to them: "From Umar ibn al-Khattab to Abu Ubaydah and Mu'adh ibn Jabal: Peace be upon you. To proceed: You have written to me mentioning that you knew me when the affair of my own self was important to me, and that I have now become the one in charge of the affair of this nation, its red and its black, and that the noble and the lowly, the enemy and the friend sit before me, and each has a share of that. You wrote: 'So look at how you will be at that time, O Umar.' And there is no power and no strength for Umar at that time except by Allah. You wrote warning me of what the nations before us were warned of. The alternation of night and day has long been bringing the appointed times of people closer, bringing every distant thing near, wearing out every new thing, and bringing every promised thing, until people arrive at their dwellings in Paradise or Hell. You wrote mentioning that you used to say that the affair of this nation would return in its final time to being brothers in public and enemies in private. You are not those people; this is not the time for that. That is a time in which desire and fear appear, where the desire of some people for others is for the sake of their worldly affairs, and the fear of some people for others. You wrote it as advice, exhorting me by Allah to place your letter in no other place than the place it descended from in your hearts, and that you wrote it [sincerely]. You have spoken the truth. So do not cease writing to me, for I have no independence from you. And peace be upon you."