What is the ruling on one who slanders the Prophet, peace and blessings be upon him?

Lying about the Prophet, peace and blessings be upon him, is a grave abomination and a great sin. A group of scholars has gone so far as to say that whoever intentionally does so commits an act of disbelief (kufr). Al-Hafiz Ibn Hajar held that lying about him is a major sin, but not like lying about others. Sheikh Al-Islam Ibn Taymiyyah supported the view that whoever lies about him verbally commits an act of disbelief, explaining that lying about him is like lying about Allah. It is considered tantamount to denying him, adding to the religion, or showing disrespect towards him. He cited as evidence the hadith (definition) of the man who claimed that the Messenger clothed him with a robe and commanded him to rule over people's wealth. The Prophet, peace and blessings be upon him, ordered him to be killed, saying: "The enemy of Allah has lied."

Some believe that one who lies about him does not commit an act of disbelief unless his lie is accompanied by something else that necessitates disbelief. Others, however, maintained that his punishment is severe without deeming him a disbeliever or ordering his execution, unless his lie contains an obvious defect indicating mockery of him, in which case he would be considered a disbeliever.