حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ بْنِ مَيْسَرَةَ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، - يَعْنِي ابْنَ مُحَمَّدٍ - أَخْبَرَنِي يَزِيدُ بْنُ عَبْدِ اللَّهِ بْنِ الْهَادِ، عَنْ مُحَمَّدِ بْنِ إِبْرَاهِيمَ، عَنْ بُسْرِ بْنِ سَعِيدٍ، عَنْ أَبِي قَيْسٍ، مَوْلَى عَمْرِو بْنِ الْعَاصِ عَنْ عَمْرِو بْنِ الْعَاصِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِذَا حَكَمَ الْحَاكِمُ فَاجْتَهَدَ فَأَصَابَ فَلَهُ أَجْرَانِ وَإِذَا حَكَمَ فَاجْتَهَدَ فَأَخْطَأَ فَلَهُ أَجْرٌ ‏"‏ ‏.‏ فَحَدَّثْتُ بِهِ أَبَا بَكْرِ بْنَ حَزْمٍ فَقَالَ هَكَذَا حَدَّثَنِي أَبُو سَلَمَةَ عَنْ أَبِي هُرَيْرَةَ ‏.‏

It was narrated that `Amr bin Al-`As said "The Messenger of Allah said: 'If a judge passes a judgment having exerted himself to arriveat what is correct, and he is indeed correct, he will have two rewards. If he passes judgment having exerted himself to arrive at what is correct, but it is incorrect, he will have one reward.'"I narrated it to Abu Bakr bin Hazm and he said: "This is what Abu Salamah narrated to me from Abu Hurairah. Translation Not Available

Book reference: Sunan Abi Dawud Book 25 Hadith 3574
Web reference: Sunan Abi Dawud Book 24 Hadith 3567