From Sahih Muslim (Hadith 4487):
Amr b. al-'As reported that he heard Allah's Messenger (ﷺ) as saying: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
Reference: Muslim 4487