Bulugh al-Maram, Book 14, Hadith 1400 (English) ⓘ

Narrated 'Amr bin al-'Aas (RA): He heard Allah's Messenger 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 )‏.‏