That he heard Messenger of Allah ﷺ saying, "If a judge gives a verdict according to the best of his knowledge and his verdict is correct (i.e. agrees with Allah and His Apostle's verdict) he will receive a double reward, and if he gives a verdict according to the best of his knowledge and his verdict is wrong, (i.e. against that of Allah and His Apostle) even then he will get a reward ." (Using translation from Bukhārī 7352)
أَنَّهُ سَمِعَ رَسُولَ اللهِ ﷺ يَقُولُ إِذَا حَكَمَ الْحَاكِمُ وَاجْتَهَدَ ثُمَّ أَصَابَ فَلَهُ أَجْرَانِ وَإِذَا حَكَمَ وَاجْتَهَدَ ثُمَّ أَخْطَأَ فَلَهُ أَجْرٌ