Musnad Ahmad 551:24

Abu al-Nadr narrated to us, he said: al-Faraj narrated to us, he said: Muhammad ibn Abd al-A’la narrated to us, from his father, from Abdullah ibn Amr, from Amr ibn al-Aas, who said: Two disputants came to the Messenger of Allah, peace and blessings of Allah be upon him, disputing with one another. He said to Amr: "Judge between them, O Amr." He said: "You are more entitled to that than I am, O Messenger of Allah." He said: "Even so." He said: "If I judge between them, what is for me?" He said: "If you judge between them and you are correct in your judgment, then you have ten good deeds. And if you exert your effort and you are mistaken, then you have one good deed."