There are two sides of everyone, the left and the right. Since keeping hands on one of them is a sign of arrogance, this is prohibited in Salat. At-Tabarani and Al-Baihaqi have reported a Hadith in which Messenger of Allah (PBUH) said: "Placing the hands on the sides during Salat, is an act of the dwellers of the Fire." One can, however, do so only if there is such a pain that he is compelled to place the hands there.