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.  

أَنَّهُ سَمِعَ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا حَكَمَ الْحَاكِمُ فَاجْتَهَدَ ثُمَّ أَصَابَ فَلَهُ أَجْرَانِ وَإِذَا حَكَمَ فَاجْتَهَدَ ثُمَّ أَخْطَأَ فَلَهُ أَجْرٌ  

This hadith has been narrated on the authority of Abu Huraira with another chain of transmitters.  

عَنْ أَبِي هُرَيْرَةَ  

This hadith has been transmitted on the authority of Usama b. al-Had al-Laithi.  

حَدَّثَنِي يَزِيدُ بْنُ عَبْدِ اللَّهِ بْنِ أُسَامَةَ بْنِ الْهَادِ اللَّيْثِيُّ بِهَذَا الْحَدِيثِ مِثْلَ رِوَايَةِ عَبْدِ الْعَزِيزِ بْنِ مُحَمَّدٍ بِالإِسْنَادَيْنِ جَمِيعًا