Sayyar (may Allah have mercy on him) said: I heard al-Sha'bi (may Allah have mercy on him) say that there was a dispute between Sayyiduna Umar (may Allah be pleased with him) and Sayyiduna Ubayy (may Allah be pleased with him). Sayyiduna Umar (may Allah be pleased with him) said: "Appoint a man as arbiter (between us)." So they appointed Sayyiduna Zayd bin Thabit (may Allah be pleased with him) as arbiter. The narrator said: They came to him. Sayyiduna Umar (may Allah be pleased with him) said: "We have come to you so that you may judge between us, and the arbiter is come to in his own house." When they entered upon him, he seated Sayyiduna Umar (may Allah be pleased with him) on his bed. Sayyiduna Umar (may Allah be pleased with him) said: "This is the first injustice you have committed in your judgement; seat me and my opponent in the same place." The narrator said: They related the matter to him, and Sayyiduna Zayd (may Allah be pleased with him) said to Sayyiduna Ubayy (may Allah be pleased with him): "The oath is upon the Commander of the Faithful, but if you wish, you may excuse him." Then Sayyiduna Umar (may Allah be pleased with him) took the oath. Then Sayyiduna Umar (may Allah be pleased with him) said to Sayyiduna Zayd (may Allah be pleased with him): "You will not be fit to be a judge until I and an ordinary man are equal in your eyes."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20510