Musnad Ahmad 30:3340

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: A funeral procession passed by the Messenger of Allah, may Allah bless him and grant him peace, and they praised it with good, mentioning its good qualities. The Messenger of Allah, may Allah bless him and grant him peace, said: "It has become incumbent." Then another funeral procession passed by him, and they praised it with evil, mentioning its evil qualities. The Messenger of Allah, may Allah bless him and grant him peace, said: "It has become incumbent. You are the witnesses of Allah upon the earth."