Musnad Ahmad 30:2946

'Abd al-Rahman narrated to us, from Sufyan, from Ibrahim ibn 'Amir, from 'Amir ibn Sa'd, from Abu Hurayrah, that a man who had died was mentioned in the presence of the Prophet, may God bless him and grant him peace. They spoke well of him and praised him, so the Messenger of God, may God bless him and grant him peace, said: "It has become incumbent." Then another man was mentioned in his presence, and they spoke ill of him and criticized him, so the Prophet, may God bless him and grant him peace, said: "It has become incumbent." He said: "You are witnesses, some of you over others."