All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is disliked to place one's hands on his waist during the prayer. The waist is the middle of the human body, and it is the narrow section above the hips.
The Hanafi scholar As-Sarkhasi said: "He should not place his hands on his waist, as it is reported from the Prophet, sallallaahu ‘alayhi wa sallam, that he forbade this action in prayer."
The Maaliki scholar Al-Muwaaq quoted ‘Iyadh as saying: "From the disliked actions in prayer is to place one's hands on his waist while standing, as it is from the actions of the Jews."
The Hanbali scholar Al-Buhooti said: "It is disliked that one places his hands on his waist during the prayer, as it is narrated on the authority of Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam, forbade man to place his hands on his waist during the prayer." [Muslim and others]
Allaah Knows best.