Musnad Ahmad 541:14

Abdullah ibn Yazid narrated to us, Haywah narrated to us, Yazid ibn Abdullah ibn al-Had narrated to me, from Muhammad ibn Ibrahim ibn al-Harith, from Busr ibn Sa’id, from Abu Qays, the freed slave of Amr ibn al-As, from Amr ibn al-As, that he heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "When a judge judges, and he exerts effort (ijtihad) and is correct, then he has two rewards. And when he judges, and he exerts effort and is mistaken, then he has one reward." He (Amr) said: So I narrated this hadith to Abu Bakr ibn Amr ibn Hazm. He said: "Thus did Abu Salamah ibn Abd al-Rahman narrate to me, from Abu Hurayrah."