Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Placing hands on the side (hip) during prayer is the way of comfort and relaxation of the people of Hell."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المكروهة فى الصلاة التى قد نهي عنها المصلي / 909