Narrated 'Amr bin al-'Aas (RA)

He heard Allah's Messenger (ﷺ) say, "When a judge gives a ruling, having tried his best to decide correctly, and is right (in his decision), he will have a double reward; and when he gives a ruling having tried his best to decide correctly, and is wrong (in his decision), he will have a single reward." [Agreed upon].