Sahih Muslim | Book 18 | Hadith Number 4261

Narrated by 'Amr b. al-'As
'Amr b. al-'As reported that he heard Allah's Messenger(may peace be upon him) as saying: When a judge gives a decision, having tried his best to decide correctly and is right, there are two rewards for him; and if he gave a judgment after having tried his best (to arrive at a correct decision) but erred, there is one reward for him.