If a judge issues a ruling, having tried his best to decide correctly, and his ruling is right, he will have a double reward, but if he issues a ruling, having tried his best to decide correctly, and his ruling is wrong, he will have a single reward. [Al-Bukhari]

ذَا حَكَمَ الحَاكِمُ فَاجْتَهَدَ ثم أَصابَ فله أَجْرَان، وإِذا حَكَمَ فَاجْتَهَدَ ثم أَخْطَأَ فله أَجْرٌ». [صحيح.] - [متفق عليه.]