كتاب المنثورات والملح
19
The Book of Miscellaneous ahadith of Significant Values
'Amr bin 'Al-'As (May Allah be pleased with him) said:
I heard the Messenger of Allah (ﷺ) saying, "When a judge utilizes his skill of judgement and comes to a right decision, he will have a double reward, but when he uses his judgement and commits a mistake, he will have a single reward."

[Al-Bukhari and Muslim]

وعن عمرو بن العاص رضي الله عنه سمع رسول الله صلى الله عليه وسلم يقول‏:‏ ‏ "‏إذا حكم الحاكم، فاجتهد، ثم أصاب، فله أجران، وإن حكم واجتهد، فأخطأ، فله أجر‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏
Sunnah.com reference : Book 19, Hadith 49
Arabic/English book reference : Book 19, Hadith 1856