Maliks Muwatta Book 36, Hadith Number 2.
Section : Stimulation of Desire to Judge Correctly.
Malik related to me from Yahya ibn Said from Said ibn al-Musayyab that Umar ibn al-Khattab had a dispute brought to him between a muslim and a jew. Umar saw that the right belonged to the jew and decided in his favour. The jew said to him, “By Allah! You have judged correctly.” So Umar ibn al-Khattab struck him with a whip and said, “How can you be sure.” The jew said to him, “We find that there is no judge who judges correctly but that there is an angel on his right side and an angel on his left side who guide him and give him success in the truth as long as he is with the truth. When he leaves the truth, they rise and leave him.”