Musannaf Ibn Abi Shayba 17:543

Marwan ibn Mu'awiyah narrated to us, from Muhammad ibn Suqah, who said: I came to Nu'aym ibn Abi Hind. He brought out a scroll to me, and in it was: "From Abu Ubaydah ibn al-Jarrah and Mu'adh ibn Jabal to Umar ibn al-Khattab: Peace be upon you. As for what follows..." Then he wrote to them from Umar ibn al-Khattab to Abu Ubaydah ibn al-Jarrah and Mu'adh ibn Jabal: "Peace be upon you both. As for what follows..."