Abu Huraira (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) forbade praying with one's hand placed on the back.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1466
(Explanation of Hadith 1465) Placing the hands on the waist is referred to as "khusr," which is prohibited during prayer because it is a sign of arrogance. One opinion is that the inhabitants of Hell will rest in this manner, or that Iblis was cast down from the heavens in this state, or that the Jews used to do this. And Allah knows best.