Musnad Abu Ya'la al-Mawsili 129:137

On the authority of Anas bin Malik, he said: A funeral procession passed by the Prophet, 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, and they praised it with evil. 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."