Musnad Ahmad 30:2883

Waki’ narrated to us, from Sufyan and Mis’ar, from Ibrahim ibn ‘Amir ibn Mas’ud al-Jumahi—Sufyan said: from ‘Amir ibn Sa’d, and Mis’ar said: I think it is from ‘Amir ibn Sa’d—from Abu Hurayrah, who said: A funeral procession passed by the Prophet, may Allah bless him and grant him peace, and they praised it with good. He said, "It has become incumbent." Then another funeral procession passed by him, and they praised it with evil. He said, "It has become incumbent." They said, "O Messenger of Allah, what is 'it has become incumbent'? What is 'it has become incumbent'?" He said, "Some of you are witnesses over others."