Silsilat al-Ahadith al-Sahihah

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates: Some people passed by the Prophet (peace and blessings be upon him) carrying a funeral bier. The people spoke well of the deceased. The Prophet (peace and blessings be upon him), upon hearing this, said: "It has become obligatory." Then another funeral bier was carried past, and the people spoke ill of the deceased. The Prophet (peace and blessings be upon him) said: "It has become obligatory." Then he said: "You are witnesses over one another."