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].
Reference: Bulugh al-Maram 1400
In-book reference: Book 14, Hadith 4
English translation: Book 14, Hadith 1400