Is burning the deceased's body permissible in Islam, knowing that it is a family custom of a new Muslim, and he thought it was the best way to preserve the environment?

The teachings of Islam came to honor humankind. The Prophet, peace and blessings be upon him, clarified what pertains to the treatment of the deceased. Islam has legislated for the deceased Muslim ritual washing (ghusl), purification, and shrouding in white garments, followed by offering the funeral prayer for them and supplicating for mercy and forgiveness, then burying and concealing them in the earth. It is also legislated to visit graves and supplicate for their occupants.

In contrast, it is not permissible to cremate the deceased in Islam, because it is a desecration of their dignity. Islam has forbidden stepping on or sitting upon a grave. Even non-Muslims are legislated to be buried, and Muslims do so for those non-Muslims who die in their lands, provided that disbelievers are not buried with Muslims in the same place. Some disbelievers may cremate the bodies of their dead due to their disbelief in the Hereafter. The obligation in Islam is to bury the deceased, not to cremate them.