Musannaf Ibn Abi Shayba 27:366

Abu Mu'awiyah narrated to us, from al-A'mash, from Salim, who said: The people of Najran came to Ali. They said: "O Commander of the Faithful, your document is in your hand and your intercession is with your tongue. Umar expelled us from our land, so return us to it." Ali said to them: "Woe to you! Umar was of sound judgment, and I will not change the work of Umar." Al-A'mash said: They used to say, "If he had anything against Umar in his heart, Ali would have seized this opportunity."