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. (Using translation from Muslim 1716a)
"إذا حَكَمَ الحَاكِم فَاجْتَهدَ؛ فَأصَابَ؛ فَلَهُ أَجْرَانِ، وَإِذَا حَكَمَ فَاجْتَهَدَ؛ فَاخْطَأَ فَلَه أجْرٌ وَاحِدٌ".