I was reading a book (of questionable historical accuracy) that claimed that the Muslims had a tradition that Muhammad's coffin floated in mid-air.
Upon further research I found that: "Historie van den Oorsprongck, Geslacht, Geboorte, Opvoedinge, en Leere des grooten valschen Propheets Mahomets" details this and many other myths regarding Muhammad.
Obviously this is false (I've never been to Medina and I couldn't find any great pictures inside the tomb, but I'm going to assume it's false) and I have never heard any of this from Muslims.
However was this created from thin air as just another thing to write in a Christian polemic? Or was this some sort of poorly misunderstood part of Islamic teaching some writer had heard and misinterpreted?
Sometimes the bizarre stories that circulated in Europe about Muhammad do stem from misunderstandings (or deliberately incorrect retellings) of Islamic teachings and traditions. Christians were sometimes aware of stories like the revelation of the Qur’an by the archangel Gabriel, or Muhammad’s marriages, or the “night journey” to Jerusalem, but they’re always twisted to portray Muhammad as a trickster, a heretic, or perhaps even the Antichrist. They’re also pretty rare because it requires a level of familiarity with Islam that most Christians didn’t have (and didn't care to have).
Christian stories about Muhammad and Islam are much more likely to be adaptations of Christian tales about saints and heretics, or of classical Greek and Roman myths, simply updated to insert Muhammad. There were older biographies of Muhammad written in Spain and the Byzantine Empire, which had direct contact with Islam starting in the 7th and 8th centuries. Further north in Europe in France and Germany, where there was little or no contact with Islam, there was also little or no interest in learning anything about Islam at all until the time of the crusades in the 11th/12th centuries. In this case, the story of Muhammad’s tomb is from the Vita Mahumeti, a 12th century biography of Muhammad by Embrico of Mainz, the first author (outside of Spain and Byzantium) to write about Muhammad.
His biography has a lot of other nonsense, including the well-known idea that Muhammad was epileptic (an old Byzantine legend), that God killed him for his sins, and that pigs partially devoured his dead body. Embrico apparently invented the story that Muhammad was buried in a golden tomb, which levitated through the use of magnets. This is sort of a reversal of the theme of levitation among Christian saints; for Christians it’s a sign of holiness, but for Muhammad it’s a sign of deceit and a sign of the stupidity of his followers.
The story turned out to be very popular and was included in other medieval Christian works on Islam. Sometimes there’s a levitating idol of Muhammad rather than a levitating tomb. The tomb/idol is usually located in Mecca (or Libya…or Babel), rather than where it actually is, in Medina.
So in short, it has nothing to do with any Islamic tradition, it’s simply the product of Embrico of Mainz’s lively imagination.
I've written some previous answers about how well Christians understood Islam in Embrico's time, which may also be helpful here:
Did Muslims worship a silver idol of Muhammad in Jerusalem?
I'm a Crusader heading towards the Holy Land in 1096. How much do I understand about Islam?
Sources:
John V. Tolan, "Anti-Hagiography: Embrico of Mainz's Vita Mahumeti", in Journal of Medieval History 22 (1996), pg. 25-41.
John V. Tolan, Sons of Ishmael: Muslims through European Eyes in the Middle Ages (University Press of Florida, 2008).