Because there is no evidence that the Prophet (peace and blessings of Allaah be upon him) or his companions practiced this, as well as because it involves imitating people from this world, it is not advised to bury the deceased in a coffin. And the earth more effectively absorbs his remains.
In al-Insaaf, it reads:
Even if the deceased is a woman, it is prohibited to bury her in a coffin. Imam Ahmad made this claim...READ FULL; FROM THE SOURCE.
The Shafa’i khateeb Al-Sharbeeni wrote in his book Mughni al-Muhtaaj:
According to the consensus of scholars, burying the deceased in a coffin is prohibited because it is bid’ah (an innovation), unless the ground is wet or soft, in which case it is permitted because it serves a purpose. Except in this instance, no one should follow the deceased’s instructions. Similar circumstances exist if the deceased was burned in a fire and could only be transported in a coffin.
According to al-Mawsoo’ah al-Fiqhiyyah,
According to scholarly consensus, burying the dead in a coffin is makrooh because it is against bid’ah, so you shouldn’t follow his instructions. However, if it serves a purpose—for example, if the deceased must be buried in a coffin because he was burned—it is not considered makrooh.
And Allah is the wisest.
