Narrated `Amr bin Al-`As: That he heard Allah’s Messenger (peace be upon him) 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”
Sahih al-Bukhari, Book 96, Hadith 79
