It is narrated from Amr bin Al-Aas (may Allah be pleased with him) that he heard the Messenger of Allah (ﷺ) saying: "When a ruler makes a judgment by ijtihad and is correct, he will have a double reward, and when a ruler makes a judgment by ijtihad and errs, he will have a single reward."
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1687