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