Hadith 1752
Abu Hurairah (May Allah be pleased with him) said:
عن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم: نهى عن الخصر في الصلاة. ((متفق عليه)).
We are prohibited from placing the hand on the side during As-Salat (the prayer). [Al-Bukhari and Muslim].
In plain words
This brief report tells us the believers were told not to stand in prayer with a hand resting on the hip. It guards against a relaxed, self-satisfied posture before Allah.
Reference : Riyad as-Salihin 1752 In-book reference : Book 17, Hadith 242