Musnad Ahmad 551:16

Abu Salamah narrated to us, he said: Bakr bin Mudar informed us, from Yazid bin Abdullah bin Usamah bin al-Had, from Muhammad bin Ibrahim, from Busr bin Sa'id, from Abu Qays, the freed slave of 'Amr, from 'Amr ibn al-'As, that he heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "When a judge judges and exerts effort, then he is correct, he has two rewards. And when he judges and exerts effort, then he is mistaken, he has one reward."