Why Prostration in Prayers must be done on Earth

Abu Sa'eed used to relate that he had seen the Prophet doing his prostration (Sijdah ) on wet mud, so much so that he could see the stains of mud on his forehead.
Saheeh Bukhari, Part I, p. 104

The Prophet used to rest his forehead on the earth (turba) while prostrating.
Saheeh Bukhari, Part 1, p. 97

The Prophet used to do his prostration on Khumrah (Sijdaghah).
(1) Saheeh Bukhari, Part 2, p. 214 & p.243
(2) Jam'a Tirmizee, p.46
(3) Kanzul Ummal, Part 2, p. 12

The Prophet declared that the best spot for prostration was the earth or a thing that grows from the earth.
Kanz-ul-Ummal Part 4. p. 113

"The Prophet said to his wife Umme Salma, "Please fetch the `Khamrah' (Sajdaghah) from the mosque." The word Khumrah means a small piece of chatai made from palmleaf on which only the head could be rested when prostrating. lbn-e-Aseer in his Jam'a-ul-Usool has written, "Khumrah is the Sijdagah on which the Shi'as of our time perform their prostration on."

"I maintain that by this tradition it is `sunnat' to keep a `sajdagah'. Those who forbid it and call it the way of the `Rafzees' are wrong. To practice this `sunnat', I often perform my prostration on a fan made from palmleaf and do not care about the criticism of the ignorant. We are concerned only with the `sunnat' of the Prophet, no matter whether they call it the way of the `Rafzees' or `Kharijees'. Let them rave about it."

Sunni `Alim Vahidul Zamankhan
Anwar-ul-Laghat, Chap. 7, p. 118