Musannaf Ibn Abi Shayba 6:1199

Muhammad ibn Bishr narrated to us, he said: Mushir narrated to us, he said: Ibrahim ibn Amir ibn Mas'ud narrated to me, from Amir ibn Sa'd, from Abu Hurayrah, who said: A man died and was mentioned in the presence of the Prophet, may Allah bless him and grant him peace, and he was praised with good. He said: "It has become incumbent." Then another died and evil was mentioned of him, and the Prophet, may Allah bless him and grant him peace, said: "It has become incumbent." Some of the people expressed wonder at the saying of the Messenger of Allah, may Allah bless him and grant him peace, "It has become incumbent." The Prophet, may Allah bless him and grant him peace, said: "You are the witnesses of Allah on the earth, one against another."