Musannaf Ibn Abi Shayba 6:1195

Zayd ibn al-Hubab narrated to us, from Musa ibn Ubaydah, from Iyas ibn Salamah, from his father, who said: The Prophet, may Allah bless him and grant him peace, passed by the funeral of a man from the Ansar, and it was praised with good. He said, "It has become obligatory." Then another funeral passed by him, and it was praised with less than that. The Messenger of Allah, may Allah bless him and grant him peace, said, "It has become obligatory." They said, "O Messenger of Allah, and what is 'it has become obligatory'?" He said, "The angels are the witnesses of Allah in the heaven, and you are the witnesses of Allah on the earth."