Praise be to Allah.
The phenomenon of incorruptibility that you asked about – which refers to the body not decomposing despite the passage of time after death – is not mentioned in any of the Islamic texts except in the case of the Messengers and Prophets only.
That is mentioned in a saheeh hadith from Aws ibn Abi Aws (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “One of the best of your days is Friday. On it Adam was created, on it he died, on it the Trumpet will be sounded, and on it all creation will swoon. So send a great deal of blessings upon me on this day, for your blessings will be shown to me.”
They said, O Messenger of Allah, how will our blessings be shown to you when your body has disintegrated?
He said, “Allah has forbidden the earth to consume the bodies of the Prophets.”
Narrated by Imam Ahmad in al-Musnad (26/840.
There is nothing in the Qur’an or Sunnah that could explain this phenomenon happening to people other than the Prophets, whether they are martyrs, righteous people or close friends of Allah (awliya’), young or old, male or female. We do not have any prophetic text that tells us about this matter or explains the reason behind it or its significance, or whether it is a sign of the righteousness of the person concerned, or whether it could happen to people who are not righteous. Allah, may He be glorified and exalted, has not told us in His Book the reason why some bodies are protected from disintegration and decomposition, whilst others are not.
As for the Messengers and Prophets, it is not possible for their bodies to decompose, because Allah, may He be glorified and exalted, has forbidden the earth to affect them in the way in which it affects other dead bodies. But this does not mean that everyone whose body is left intact is like the Messengers and Prophets in piety and righteousness. This is not a correct analogy, and it is not the right conclusion, for there may be some natural factors which lead to the preservation of the body and protect it from decomposing. All we can say with regard to this issue is that we hope that this phenomenon is a sign of righteousness and acceptance with Allah, may He be glorified, if the person in question was one of the good and righteous people.
Thus you may learn that the matter does not go any further than being a matter of hope and supplication. So if we find someone among the Muslims whose body remained intact after burial, we ask Allah, may He be exalted, to make it a sign of mercy, virtue and goodness for the deceased, but we do not believe – either on the basis of probability or certainty – that it is evidence of the righteousness and piety of the individual.
The fact that this phenomenon happens to some non-Muslims may be explained in the following manner:
It may be due to natural factors in the body itself, or in the soil (in which he is buried) or in that location. This is important research that scientists and experts could investigate and find out the scientific causes, based on a study of the composition of the soil and the types of bacteria present in it, or finding out whether some substances were applied to the body (that led to its preservation). All of these are possible causes of its being preserved. This is important research that should be carried out by scientists and researchers.
It may also be due to deceptive actions on the part of some fanatics who show the body as if it is preserved from disintegration, when in fact it is decaying and decomposing, but it appears to be intact because of clothing, filling in and application of wax to make the body appear to be intact, so as to convince people of the soundness of a particular religion or sect that the deceased used to follow. Focusing on emotional preaching and deceit, far removed from research based on reason, is a well-known tactic among many of the followers of various religions and sects, but at the end of the day it backfires and they cannot deceive people or their followers forever.
The body may indeed be intact, but there is a difference between this appearance of being intact and the body that appears fresh, as if the person only recently died. The former happens in many cases, in which the body appears as though no part of it is missing, but a single touch is sufficient to make it crumble into dust. As for cases in which the body appears fresh, and in some instances a pleasant fragrance comes from the body and sometimes blood is even seen to gush forth from it, such cases are rare, and that is what we hope may be a sign of the person’s righteousness and his honour before Allah, may He be glorified and exalted, if the person was a Muslim and righteous. But still we cannot be certain of that, and we have no shar‘i evidence to prove it. Rather it is no more than the hope that it is a sign of Allah honouring such people, as explained above.
Conclusion: we call people to research and study on the basis of reason the phenomenon of incorruptibility of some bodies in their graves, and to study the matter from all angles, both biological and theological. We have not seen in any Islamic textbook anyone who regards such phenomena as a sign that the deceased was following the right sect or religion. Similarly, we have not come across anyone among Muslim scholars who quotes that as evidence in his debates with non-Muslims, because in fact it is not evidence.
This is contrary to what the clergy want, concerning whom there is research and specialist websites, who use wax and preservatives in order to show the bodies of their leaders and saints as if they have been preserved by the power of God, and as if this is a divine message calling people to believe in Christianity or Judaism. All of that comes under the heading of exaggeration and using emotion and deception, but the church cannot fool people who possess reason.
And Allah knows best.