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)).