Majma al-Zawaid — Hadith 7002

Sayyiduna Amr bin al-As (may Allah be pleased with him) narrates: Two men came to the Prophet (peace and blessings be upon him), opposing parties in a dispute. The Prophet (peace and blessings be upon him) said to Sayyiduna Amr (may Allah be pleased with him): "O Amr, judge between the two of them." He said: "O Messenger of Allah, you are more entitled to this than I am." The Prophet (peace and blessings be upon him) said: "Even if that is so (you judge)." He said: "If I judge between them, what will I get?" The Prophet (peace and blessings be upon him) said: "If you judge between them and judge correctly, you will get ten good deeds; and if you exercise your reasoning (ijtihad) and err, you will get one good deed." (Allamah al-Haythami says:) I say: From him, in the "Sahih",
this report is narrated: "If you judge correctly, you will receive a double reward, and if you err, you will receive one reward."
Ahmad recorded it, and al-Tabarani in al-Mu'jam al-Kabir; its chain contains a narrator whom I do not know.