Why do Shias pray with their hand by their sides, and put there forehead on clay from Karbala?
A: Every thing in True Islam has a wisdom behind it. Shia's have to put their hands by their sides or in front on their thighs.
Everything are symbols and are our whole prayer is symbolic.
1)The symbol of standing in front of Allah with my hands by my side is that my whole body is my tongue/ my expression/ bayan in arabic.
Expressing my submission to him in a body language form, apart from vocal expression.
Because in the day of judgement it has been said in the Quran that those who want to protest. Allah will shut their mouths and their body starts witnessing and talking , the hands the leg/ feet start confessing.
The hands start witnessing and confessing to the thighs, and thighs to the hands.
Quran
That Day shall We set a seal on their mouths. But their hands will speak to us, and their feet bear witness, to all that they did. (Yaseen: 65)
So my Whole body, kind of becomes my tongue, my speech. So when you put your hands by your side or front thighs it becomes the symbol of your tongue and speech.
Hence the palms of the Hands must touch the thighs and create a connection.
Sunnis on the other hand, are locking their hands and it symbolizes the disconnect of the witnessing and confessing of the hands to the legs and vice versa, and the form is no more of the tongue any more. Thus disrepectful to the Law Of Allah.
-----------
2) Sajdah on natural material like leaf, stone, sand , clay, clean ground,Tissue, or even your finger if you cannot find any of the mentioned.
When we do Sajdah (prostrate) is is symbolic we are made from the earth/clay.
1-After the first sajdah when we sit it symbolizes we are given life and formed from earth.
2-When we do the second sajdah it is symbolic of we will return to the earth ( die )
3-When we sit down after that, it means we will be brought to life again/Resurrected.
Hence if you pray on silk prayer matt and put your forehead directly on the fabric, it would not symbolize we are from earth.
Turbah ( clay ) from Karbala is just a sign of respect.