Imam al-Sha'bi (may Allah have mercy on him) said: There was a dispute over something between Sayyiduna Umar (may Allah be pleased with him) and Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him). Sayyiduna Ubayy (may Allah be pleased with him) made a claim against Sayyiduna Umar (may Allah be pleased with him), and he denied it. The two of them took their dispute to Sayyiduna Zayd bin Thabit (may Allah be pleased with him) at his house. When they entered upon him, Sayyiduna Umar (may Allah be pleased with him) said: We have come to you so that you may judge between us, and the judge is to be come to in his own house. Sayyiduna Zayd (may Allah be pleased with him) made room for him at the head of his bedding and said: O Commander of the Believers! Sit here. Sayyiduna Umar (may Allah be pleased with him) said: You have been unjust in your very first ruling; rather, I will sit with my opponent. So they sat before him. Sayyiduna Ubayy (may Allah be pleased with him) made his claim and Sayyiduna Umar (may Allah be pleased with him) denied it. Sayyiduna Zayd (may Allah be pleased with him) said to Sayyiduna Ubayy (may Allah be pleased with him): Excuse the Commander of the Believers from the oath; I would not ask this for anyone other than him. Sayyiduna Umar (may Allah be pleased with him) took the oath, and then he swore: Zayd bin Thabit will not attain (true) judgeship until Umar and an ordinary man of the Muslims are equal in his sight.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20463