عن أبي هريرة رضي الله عنه
أن رسول الله صلى الله عليه وسلم: نهى عن الخصر في الصلاة
We are prohibited from placing the hand on the side during As-Salat (the prayer).
Riyad as-Salihin 1752, Book 18, The Book of the Prohibited actions, Hadith 242. 1 min read