Hadith 2286
Sayyiduna Abu Huraira (may Allah be pleased with him) narrates this statement of the Prophet (peace and blessings of Allah be upon him): "Placing the hand on the side during prayer is a way of seeking comfort used by the people of Hell." (Imam Ibn Hibban, may Allah have mercy on him, says: What is meant by this is that this is the practice of the Jews and Christians, and those people are the people of Hell.)