Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 338
Prohibition of Placing the hands on the sides during As-Salat (The Prayer)

1752. Abu Hurairah (May Allah be pleased with him) said: We are prohibited from placing the hand on the side during As-Salat (the prayer).
[Al-Bukhari and Muslim].

Commentary:  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.