Abu Hurairah (May Allah be pleased with him) said:
We are prohibited from placing the hand on the side during As-Salat (the prayer).

[Al-Bukhari and Muslim].

عن أبي هريرة رضي الله عنه أن رسول الله ﷺ: نهى عن الخصر في الصلاة. ((متفق عليه)).