Musnad Ahmad 32:894

Yahya ibn Sa'id narrated to us, from Humayd, from Anas, that a funeral passed by the Prophet, may Allah bless him and grant him peace, and it was spoken of with good, and tongues followed one another with good for it. The Prophet, may Allah bless him and grant him peace, said: "It has become incumbent." Then another funeral passed by, and they spoke of it with evil, and tongues followed one another with evil for it. The Prophet, may Allah bless him and grant him peace, said: "It has become incumbent. You are the witnesses of Allah upon the earth."