Salim bin Abi al-Ja'd said: If Sayyiduna Ali (may Allah be pleased with him) had ever criticised Sayyiduna Umar (may Allah be pleased with him), he would have done so on the day the people of Najran came to him. Sayyiduna Ali (may Allah be pleased with him) had written the document between the people of Najran and the Prophet (peace and blessings be upon him). In the time of Sayyiduna Umar (may Allah be pleased with him) their numbers grew, until Sayyiduna Umar (may Allah be pleased with him) feared harm from them to the people, and disagreement arose among them. They came to Sayyiduna Umar (may Allah be pleased with him) and asked for a change, so he made the change at their request. Then they regretted it, or disagreement arose between them; then they sought to have the matter reversed, but it was refused. When Sayyiduna Ali (may Allah be pleased with him) became the Commander of the Believers, they came to him and said: O Commander of the Believers! Your intercession by your tongue, and the document in your own handwriting! Sayyiduna Ali (may Allah be pleased with him) said: Woe to you! Sayyiduna Umar (may Allah be pleased with him) was a man of sound judgement in affairs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20375