Stages of trials for the deceased in his grave and du‘aa’ for him at that time
Question

after the 3 day mourning period some muslims do 10 day prayer 20 day prayer and 30 day prayer and lastly 40 day...
i heard that the body goes through different level of trials during each of this days eg 3 days head explode 10 days stomach 20 .......on the 40th day teh whole body explodes is this true
Praise be to Allah.Firstly: 
In the answer to question no. 157114 we stated that offering du`a for the deceased after his burial is Sunnah, because of the report narrated by Abu Dawood (3221) from the hadith of ‘Uthmaan ibn ‘Affaan (may Allah be pleased with him) who said: When the Prophet (blessings and peace of Allah be upon him) had finished burying a deceased person, he would stand over him and say: “Pray for forgiveness for your brother, and ask that he be made steadfast, for he is being questioned now.” Classed as saheeh by Shaykh al-Albaani in Saheeh Abi Dawood. 
Secondly: 
Offering du`a for the deceased Muslims in general, and for close relatives in particular, is also prescribed in general terms at any time, without specifying any particular time, because of the general meaning of the words of Allah, may He be exalted (interpretation of the meaning):
“And those who came after them say: "Our Lord! Forgive us and our brethren who have preceded us in Faith”
[al-Hashr 59:10];
And the words of Ibraaheem (peace be upon him):
“Our Lord! Forgive me and my parents, and (all) the believers on the Day when the reckoning will be established” [Ibraaheem 14:41]
And the words of Nooh (peace be upon him):
“My Lord! Forgive me, and my parents, and him who enters my home as a believer, and all the believing men and women” [Nooh 71:28].
And Allah, may He be glorified and exalted, said to His Prophet Muhammad (blessings and peace of Allah be upon him):
“and ask forgiveness for your sin, and also for (the sin of) believing men and believing women” [Muhammad 47:19]. 
See also the answer to question no. 140798  
Secondly: 
Waiting for three days, then offering du`a for the deceased for a period of ten days, or twenty or thirty or forty days, on the grounds that his body goes through a number of stages of trials during this period, is an act that is contrary to the Sunnah and is a claim that is devoid of any evidence; it is a fabricated innovation. 
With regard to its being contrary to the Sunnah:
That is because the Sunnah is what is mentioned above, which is to offer du`a for the deceased at the grave immediately after his burial. Moreover offering du`a for the Muslim dead in general and some in particular is something that is recommended in general terms, without specifying any particular period or time. 
With regard to what is mentioned being a mere claim that is devoid of any evidence:
That is because these words are not known in the Sunnah, and we do not know any of the early generations or the imams and scholars of the Muslims who mentioned that, and we do not know of any doctors or coroners who have mentioned it in the modern era. 
The idea that the head of the deceased explodes in the first three days, and his stomach explodes after ten days, and his entire body explodes after forty days, is a false notion and a fabricated claim for which there is no evidence. Perhaps this idea is the basis for the innovations surrounding the fortieth day after death, when tents are set up, and people come to offer condolences, and the reciter recites the Quran. The scholars have stated that these acts are innovations (bid‘ah). 
Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: 
What is the ruling on observing the fortieth day after death, during which Quran is recited and people gather to offer condolences? 
He replied: This is one of the innovations that are practised by some people. When forty days have passed since the death, they organise a condolence gathering in which people gather at the house of the deceased, and they read Quran and illuminate in the place. In fact this comes under the heading of renewing grief, which is prohibited. "(Fatawa Noor ‘ala ad-Darb 9/2) 
The forty-day commemoration is a pharaonic custom, as has been explained previously in the answer to question no. 12552 
People have various customs and traditions in that regard, all of which come under the heading of innovations that are prohibited. 
Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: 
When seven days have passed since the death, the women of the bereaved family go to him in the graveyard, and they start to weep again. When fifteen days have passed, they repeat the same action, and again when forty days have passed, they mourn for him for a year or more, and they do not let children play or be happy. Is this permissible or not? 
He replied: 
This action is not permissible, because for women to visit graveyards, if the woman goes out of her house for this purpose, then she is cursed – Allah forbid. The same applies to morning for a whole year; it is a reprehensible act that is not permissible."(Fatawa Noor ‘ala ad-Darb (9/2) 
By the same token, if the three-day period mentioned is also something that people do, whereby they set up tents for receiving condolences for their loss for three days, during which they recite Quran and receive condolences, this is also an innovation. 
With regard to this sequence of du`as and prayers for forgiveness for the deceased being an innovation:
That is because it is something that has been introduced, for which there is no basis. We do not know of any of the Muslim scholars who suggested this, let alone there being any basis for it in the Sunnah or the acts of the Sahaabah. 
Fourthly: 
With regard to the stages through which the body passes after burial, this is something that only Allah knows. Bodies vary in the stages of disintegration, and not all bodies disintegrate in the same manner or at the same time. We have no need to know about that or examine the matter. Whether the body disintegrates rapidly or slowly, and whether the stomach explodes after ten days, or more or less than that, focusing on this and similar matters is a distraction from that which is much more important to focus on, which is the state of the soul which is either in a state of bliss or torment. 
You may find a body that has been buried for many years but has not disintegrated because of environmental factors, but the individual is in a state of humiliating punishment. 
This is Pharaoh of whom Allah says (interpretation of the meaning):
“So this day We shall deliver your (dead) body (out from the sea) that you may be a sign to those who come after you” [Yoonus 10:92]. 
Or you may find that a body has disintegrated completely, but the individual is one of the martyrs who are in a state of bliss. 
It was narrated by al-Faakihi in Akhbar Makkah (2/351) and Ibn Hazm in al-Muhalla (1/42) from Mansoor ibn ‘Abd ar-Rahmaan that his mother said: When Ibn az-Zubayr (may Allah be pleased with him) was crucified, Ibn ‘Umar (may Allah be pleased with him) entered the mosque – and that was when Ibn az-Zubayr was killed and crucified. It was said to him: Asma’ bint Abi Bakr (may Allah be pleased with her) is in a corner of the mosque. He went to her and offered her condolences and said: These dead bodies are nothing; rather the souls are with Allah, may He be exalted, so fear Allah and be patient. She said: What is there to keep me from being patient when the head of Yahya ibn Zakariya was given to one of the prostitutes of the Israelites?
Its isnaad is saheeh. 
Ibn ‘Umar went to the side of some graves that had collapsed and looked at one of the graves, where he saw a skull visible. He instructed a man to cover it with earth, then he said: These bodies will not be harmed by this dust at all; rather it is the souls that are being punished or rewarded until the Day of Resurrection."(Tafseer Ibn Rajab  2/102) 
The basic principle is that punishment or bliss in the grave happen to the soul, and the soul may be connected to the body which thus feels something of that punishment or bliss. 
See the answer to question no. 47055
To sum up: 
The details mentioned in the question about the disintegration of the body after death is an idea for which there is no evidence. 
What is mentioned about offering du`a for the deceased forty days after his death in the manner mentioned is a reprehensible innovation and it is not permissible to do that. 
And Allah knows best.