Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1113
Abu Huraira reported from the Apostle of Allah (may peace be upon him) that he forbade keeping one's hand on one's waist while praying, and in the narration of Abu Bakr (the words are): The Messenger of Allah (may peace be upon him) forbade to do so.