Riyad as-Saliheen 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 ﷺ: نهى عن الخصر في الصلاة. ((متفق عليه)).
Classification
Sahih (Authentic)
References
Riyad as-Saliheen 1752
Riyad as-Saliheen Book of Prohibited actions, Hadith 242