Mukhtasar Bukhari

Narrated Anas bin Malik (may Allah be pleased with him): People passed by with a funeral, and the Companions (may Allah be pleased with them) praised the deceased. The Prophet (peace and blessings of Allah be upon him) said: "It has become obligatory." Then another funeral passed by, and they spoke ill of the deceased. The Prophet (peace and blessings of Allah be upon him) said: "It has become obligatory." So Umar bin Khattab (may Allah be pleased with him) asked: "What has become obligatory?" The Prophet (peace and blessings of Allah be upon him) said: "Paradise has become obligatory for the one whom you praised, and Hell has become obligatory for the one whom you spoke ill of. You are Allah's witnesses on earth."