O ye who believe! If a wicked person comes to you with any news, ascertain the truth, lest ye harm people unwittingly, and afterwards become full of repentance for what ye have done. (49:6)
Ibn Mas`ûd relates that the Prophet (peace be upon him) said: “Should I tell you what is a blatant lie? It is a rumor circulating among the people.” [Sahîh Muslim (2606)]
Here the Prophet (peace be upon him) is telling us how evil it is to spread rumors. The Arabic word for rumor here is namîmah. It is a kind of rumor that is meant to foster animosity between people. Al-Râghib al-Isfahânî defines the word namîmah as follows [al-Nihâyah (5/120)]:
It is to relate to people what others say about them in order to bring about problems. It is said that this rumor-mongering also includes divulging secrets and exposing about people what they wish to keep concealed.
It is a serious sin to go back and forth between people with rumors of what they are saying about each other and what they are scheming to do. Hudhayfah heard about a man who was spreading rumors and he said: “I heard Allah’s Messenger say: ‘A rumor-monger will not enter Paradise’.” [Sahîh al-Bukhârî (6056) and Sahîh Muslim (105)]