Abu Hurairah (RA) reported from the Apostle of Allah (ﷺ) that he forbade keeping one's hand on one's waist while praying, and in the narration of Abu Bakr (RA) (the words are): The Messenger of Allah (ﷺ) forbade to do so.
Hadith ReferenceSahih Muslim / The Book of Mosques and Places of Prayer / Hadith: 545