Riyad as-Salihin 1752
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 صلى الله عليه وسلم‏:‏ نهى عن الخصر في الصلاة‏.‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏