Idris al-Awdi (may Allah have mercy on him) said: Sa'id bin Abi Burdah (may Allah have mercy on him) brought out a letter and said: This is the letter of Sayyiduna Umar (may Allah be pleased with him) to Sayyiduna Abu Musa (may Allah be pleased with him): "After praise (of Allah): Judging is a most important office and a practice that is to be followed. Understand when a case is presented to you, and spare no effort in enforcing the truth. Treat people equally before you in your gathering, so that a noble man does not hope for your leniency, and a weak man does not fear your injustice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20460