Musnad Ahmad 28:279

Hasan narrated to us, Ibn Lahi'ah narrated to us, Al-Harith ibn Yazid narrated to us, from Salamah ibn Aksum, who said: I heard Ibn Hujayrah asking Al-Qasim ibn al-Barhi: "How did you hear Abdullah ibn Amr ibn al-As reporting?" He said: I heard him saying: Two disputants brought a dispute to Amr ibn al-As, and he judged between them. The one against whom the judgment was made was displeased, so he went to the Messenger of Allah (peace and blessings of Allah be upon him) and informed him. The Messenger of Allah (peace and blessings of Allah be upon him) said: "When a judge judges and exerts effort (ijtihad) and is correct, he has ten rewards; and when he exerts effort and is mistaken, he has one reward or two rewards."