Musnad Ahmad 551:20

Abu Sa’id narrated to us, ‘Abdullah ibn Ja’far narrated to us, he said: Yazid ibn ‘Abdullah narrated to us, from Muhammad ibn Ibrahim, from Busr ibn Sa’id, from Abu Qays, the freed slave of ‘Amr ibn al-‘As, from ‘Amr ibn al-‘As, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When a judge judges and exerts his effort (ijtihad) and is correct, he has two rewards. And if he errs, he has one reward." Yazid said: I mentioned that to Abu Bakr ibn Hazm, and he said: Abu Salamah narrated it to me in this way, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), the like of it.