Sayyiduna Amr bin al-As (may Allah be pleased with him) narrates: He heard the Prophet Muhammad (peace and blessings be upon him) say: "When a judge exercises ijtihad (independent reasoning) while making a decision and gives a correct verdict, he receives two rewards, and when he exercises ijtihad and makes a mistake, he receives one reward."
Hadith Referenceصحیح ابن حبان / كتاب القضاء / 5061
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطحديث صحيح