Bulugh al-Maram 1400
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].
وَعَنْ عَمْرِو بْنِ الْعَاصِ ‏- رضى الله عنه ‏- أَنَّهُ سَمِعَ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-يَقُولُ: { 
"إِذَا حَكَمَ اَلْحَاكِمُ, فَاجْتَهَدَ, ثُمَّ أَصَابَ, فَلَهُ أَجْرَانِ.‏ وَإِذَا حَكَمَ, فَاجْتَهَدَ, ثُمَّ أَخْطَأَ, فَلَهُ أَجْرٌ" }  مُتَّفَقٌ عَلَيْهِ  1‏ .‏‏1 ‏- صحيح.‏ رواه البخاري ( 7352 )‏، ومسلم ( 1716 )‏.‏