Musnad Ahmad 32:1628

'Affan narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas ibn Malik, that a funeral procession passed by the Messenger of Allah, may Allah bless him and grant him peace, and they praised it with good. 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. The Messenger of Allah, may Allah bless him and grant him peace, said: "It has become incumbent." Then he said: "You are the witnesses of Allah upon the earth."