Salaam Alaikum. When talking about the Resurrection of human beings to be judged by God, Muhammad stated that when we die, our whole body will decay, except for one bone. He claimed that the tailbone (or the bone at the end of it) will remain, everything else will be decayed by the earth. This statement is quoted in several authentic hadiths, from Bukhari and Muslim.
(1) Narrated Al--Amash: Abu Huraira said, "Allah's Apostle said, 'Between the two sounds of the trumpet, there will be forty." Somebody asked Abu Huraira, "Forty days?" But he refused to reply. Then he asked, "Forty months?" He refused to reply. Then he asked, "Forty years?" Again, he refused to reply. Abu Huraira added. "Then (after this period) Allah will send water from the sky and then the dead bodies will grow like vegetation grows, There is nothing of the human body that does not decay except one bone; that is the little bone at the end of the coccyx of which the human body will be recreated on the Day of Resurrection." (See Hadith No. 338) (Book #60, Hadith #457)
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Between the two blowiings of the trumpet (there would be an interval of forty). They said: Abu Huraira, do you mean forty days? He said: I cannot say anything. They said: Do you mean fortv months? He said: I cannot say anything They said: Do you mean forty years? He said: I cannot say anything. Then Allah would cause the water to, descend from the sky and they (people) will sprout like vegetable The only thing in a man which would not decay would be one bone (spinal chord) from which the whole frame would be reconstituted on the Day of Resurrection.
Abu Huraira reported Allah's Messenger (may peace be upon him) as say- ing: The earth would consume every son of Adam except his spinal chord from which his body would be reconstituted (on the Day of Resurrection).
Abu Huraira reported so many ahadith from Allah's Apostle (may peace be upon him) and amongst these one was this that Allah's Messenger (may peace be upon him) said: There is a bone in the human being which the earth would never consume and it is from this that new bodies would be reconstituted (on the Day of Resurrection). They said: Allah's Messenger, which bone is that? Thereupon he said: It is the spinal bone.
Hadith (Hadis) Books (Sahih Al-Bukhari, Sahih Muslim, Sunan Abu-Dawud, and Malik's Muwatta)
According to Muhammad, every dead person is decayed by the earth, but what remains is her/her tailbone.
Muhammad stated that when every son of Adam (or person) dies, the earth will dissolve everything of the body but the tailbone. The tailbone is one of the bones in the human body that is least likely to dissolve. The website "Quran and Science" does a great job pointing this out.
The Coccyx
The site claims, as Muhammad did, that the coccyx does not get dissolved or eaten away by the earth. Often, it remains when all other parts of the body have been dissolved/acidified.
However, at other times, it certainly doesn't.
We know that in the sea, whale skeletons are eaten by worms, who dissolve them using acids.
The wispy worms may significantly speed up the whales’ decomposition, Katz said. Researchers previously thought it took decades for a skeleton to break down in the ocean, she said, but the worms consume it in less than a decade.
STUDY: BONE WORMS USE ACID TO DISSOLVE WHALE SKELETONS | UTSanDiego.com
Obviously, if this happens to a whale skeleton at sea, it also happens to human skeletons. The sons of Adam who drowned in the sea will not have spinal chords left from which their bodies will be reconstituted.
If we want to examine bodies that are buried in the earth, we will also see that often, the whole skeleton, not only the spinal chord, is eaten away.
The ship at Sutton Hoo is believed to have contained a body of a king, who was buried sometime in the Middle Ages. There was left nothing of either his body or skeleton, because the acidic acid dissolved all of it.
British Museum - Who was buried at Sutton Hoo?
No trace of a body was found during the 1939 excavation of the Sutton Hoo ship-burial.
Analyses of soil samples for residual phosphate (a chemical left behind when a human or animal body has completely decayed away), taken in 1967 during the British Museum's excavations, support the idea that a body was originally placed in the burial chamber, but that this had totally decayed in the highly acidic conditions at the bottom of the ship.
British Museum - Who was buried at Sutton Hoo?
In 2011, the remains of a young boy who died on the Titanic and was subsequently buried, were exhumed so he could be recognized. He was initially incorrectly identified as being Finnish. No spinal chord remained, but instead there was a fragment of an armbone, and teeth.
Following exhumation of the grave in 2001, which produced in a 2.4-inch-long fragment of an arm bone and three teeth, Parr and colleagues concluded that the child was Eino Viljami Panula, a 13-month-old Finnish infant who drowned with his parents in the disaster.
Titanic's 'Unknown Child' Identified : Discovery News
In Germany and some other parts of Western Europe, archaeologists find bodies that were buried in the peat bogs. Often they were of people who were drowned or executed there in other ways, hundreds of years before Christ. The soil in the bogs has acid concentration which interestingly dissolves the bones, but leaves the rest of the body intact.
Mummification in Bogs
The word mummification usually conjures an image of an ancient Egyptian that has been wrapped in gauze and preserved for thousands of years. However there is a process whereby the forces of nature cause natural mummification. This natural mummification can occurs in several climates. One of these climates is the peat bog; most peat bogs suitable for providing the proper conditions for mummification can be found in Europe. Mummification occurs because of the conditions produced by the bog. Sphagnum moss is the primary component of any peat bog; sphagnum moss grows upwards and completely encases bodies within the bog. When this sphagnum dies it condenses and forms peat, which encases the bodies in a cold, acidic, and oxygen deprived environment. The bacteria and fungi that are responsible for decomposition cannot survive without oxygen, thus the body cannot break down. However the aforementioned acidic conditions have an effect on the preserved bodies. These acidic conditions break down the bones within the bodies while leaving the skin intact. This is why bog mummies have a different appearance than their skin and bones Egyptian counterparts. The skin of a bog mummy is tanned and leather, and due to the absence of bones a bog mummy can be compared to a sack of skin.
First Year Biology | University of British Columbia – Vancouver
Most of the bodies found in the bogs had skin remaining on them but the bones were eaten away... in some cases, nothing of the bones remained.
Damendorf Man died about 300 B.C. and was discovered thousands of years later in the Eckernförde district of Germany. Unlike some bog bodies, he was flattened by the weight of the peat. Only his skin, nails, and hair (as well as his leather belt and shoes) were preserved.
As P. V. Glob wrote in The Bog People, "The rest of him has completely disappeared as if by magic. A split nearly an inch long in the region of the heart may indicate how he was killed."
http://www.mummytombs.com/bog/damendorf.htm
Although the cocyxx is a strong bone, it also eventually gets decayed in the earth, and disappears. Sometimes this happens to it after it happens to other bones, other times before.
Clearly, Muhammad's claim that the earth dissolves everything of the human body and leaves only the spinal bone is not true.
God obviously did not tell him this, because God would have known that the earth can and does eventually decay human remains, including bones.
Muhammad was a smart man and he did many good things. However, he was not a prophet of God.