189 – Abu Hurayrah (may Allah be pleased with him) reported: “The Messenger of Allah ﷺ forbade a man to pray while mukhtaṣir (arm akimbo).” Agreed upon; the wording is Muslim’s (1). Its sense is that he places his hand on his side.
In al-Bukhari, ʿĀʾishah (may Allah be pleased with her) said: “That is how the Jews pray” (2).

Glossary of the ḥadith
• mukhtaṣir: the active participle of ikhtiṣār, meaning placing one’s hand on one’s side—or both hands on both sides. A person’s khāṣirah (“side”) is the area between the hip and the lower ribs; each individual has two such sides.

Points derived from the ḥadith
1 – It is prohibited for a worshipper to pray with his hand on his side, the region between the top of the hip and the lower ribs.
2 – The wisdom behind the prohibition is to avoid resembling the Jews, for they place their hands on their sides during prayer.
3 – Another explanation is that it is the posture of the arrogant. The two rationales do not conflict, for arrogance, contempt for others, and the claim of being “God’s chosen people” are among the traits historically attributed to the Jews.
4 – What is required in prayer is humility and submissiveness, for the worshipper stands before Allah in utter lowliness, far removed from the marks and manners of the proud.