All the prophets preached Islam, the religion of nature although every prophet would have used the word in the language of his nation, instead of the Arabic word Islam . The true Sanatan Dharma or Svadharm a preached burial of the dead as it is according to nature. The burning of millions of men requires a huge quantity of firewood which creates ecological imbalance. The fumes add to pollution. The electrical pyres also are not the solution, as they consume energy, much needed by the living.
Burial is echo friendly, pollution-free and energy saving.
I never issued a fatwa on the subject. I only advised the enquirer to benefit from the experience of a number of acclaimed saints. My recommendation was also based on my own experience. You may refer to the book Kalki Avtar Aur Muhammad Sahib authored by Pt. Ved Prakash Upadhyay, in Hindi.