Musannaf Ibn Abi Shayba 13:3005

Abu Bakr narrated to us, he said: Yazid ibn Harun narrated to us, he said: Hammad ibn Salamah narrated to us, from Qatadah, that Abu Musa al-Ash'ari said: "It is not appropriate for a judge to judge until the truth becomes clear to him, just as the night is distinguished from the day." He said: This reached Umar, and he said: "Abu Musa has spoken the truth."