Musnad Abu Ya'la al-Mawsili 155:63

Muhammad ibn Mahdi narrated to us, Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Thawri, from Yahya ibn Sa'id, from Abu Bakr ibn Muhammad ibn 'Amr ibn Hazm, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When a judge judges and exerts effort (ijtihad) and is correct, he has two rewards. And when he judges and exerts effort and is mistaken, he has one reward."