Ayatollah Sistani says in his risalat 1054. A person offering prayers should perform two sajdahs after the Ruku, in each Rak'at of the obligatory as well as Mustahab prayers. Sajdah means that one should place one's forehead on earth in a special manner, with the intention of humility (before Allah). While performing Sajdahs during prayers, it is obligatory that both the palms and the knees, and both the big toes are placed on the ground.
we also read in the books of fiqh we see that if you need to pray in a najis area that you should at least make sure the place where the forehead is placed in sujud is tahir.
We see in narrations that the prophet (s) was described as having mud on his forehead, meaning he prostrated his forehead on earth.
These things all emphasize on the forehead, not in other parts. If it were necessary for your knees and feet to be placed on turbahs, for example, then we would need to pray in shorts and without socks so as to to let those parts touch. We know that the customary dress of the people who were Muslims at that time wore long dishdasha's so they would not be able to put their knees on turbah.