Nasai

891. Commentary:

➊ During the performance of each pillar (rukn) of the prayer (salah), there is a designated position for the hands. Placing the hands on the hips violates the original prescribed posture, and for this reason, it is prohibited. It has been said that Satan stands in this manner, or that the Jews used to worship in this way, or that people afflicted by calamities stand like this during lamentation, or that the inhabitants of Hell will stand like this in Hell, or that this is a characteristic of the arrogant. All these are similes; therefore, it has been prohibited. And Allah knows best.

➋ These are the meanings of «تخصر» according to the majority of scholars. Some have interpreted it as holding a staff for support, or reciting only a part of a surah, or not completing the bowing (ruku) and prostration (sujud), but these interpretations are weak, and they also contradict the forthcoming hadith.

Hadith Commentary:
Several reasons for this prohibition have been mentioned in the hadiths:
➊ Doing so is a sign of arrogance.
➋ The Jews often do this, and it has been prohibited due to resemblance to them.
➌ Iblis was cast down from the heavens in this state.
➍ The inhabitants of Hell will do this at times of rest, therefore it has been forbidden to do so during prayer.

Ziyad bin Subaih says that once I performed prayer standing beside Abdullah bin Umar (radi Allahu anhu). During the prayer, I placed my hand on my waist. When the prayer was finished, he said: "This is adopting the posture of the cross in prayer; the Messenger of Allah (sallallahu alayhi wa sallam) forbade this."
(Sunan Abi Dawud, Kitab al-Salat, Hadith: 903, and Fath al-Bari: 3/116)

Hadith Commentary:
That is, placing the hands on the hips has been prohibited.

The wisdom in this is that Iblis was cast down from the heavens in this very posture, and the Jews would often do this, or the inhabitants of Hell will take rest in this manner.

Therefore, it has been prohibited; it is also a sign of the arrogant.

Hadith Commentary: Benefits and Issues: (IKHTISAR FI AS-SALAH)
What is meant by this? There is a difference of opinion among the scholars of religion regarding this.
According to most, and the preferred meaning is that placing the hands on the waist (kokh) during prayer is not permissible.
Allamah Harawi said,
Ikhtisar is that one does not recite a complete surah, but recites two or four verses from the beginning or end, or recites quickly.
According to Imam Ghazali rahimahullah, it is to leave out a verse of prostration (sajdah) while reciting.
According to Allamah Khattabi,
it means to lean on a staff (stick) during prayer, and according to some, it means not maintaining moderation in the pillars of prayer, i.e., standing (qiyam), bowing (ruku‘), and prostration (sujud).
2.
There are various opinions regarding the wisdom behind placing the hands on the waist (kokh):
1.
Iblis was cast out of Paradise in this state.
2.
Iblis walks in this manner.
3.
This is the practice of the Jews.
4.
The inhabitants of Hell rest in this way.
5.
This is the attitude of the arrogant and boastful.
The third case is narrated from Aisha radi Allahu anha.

1:
This prohibition exists because it is a sign of arrogance, whereas prayer (salah) is the expression of humility and submissiveness before Allah.

947. Commentary:
The linguists have mentioned two or three meanings of «اختصار». One is to stand with the support of a staff, the second is to recite the Qur’an in a shortened form by starting from the end, or to excessively shorten (make brief) the pillars of the prayer. So the Imam (may Allah have mercy on him) has determined its meaning, and this is the correct one. See also: [باب۔ 155۔ 156۔ حديث903]

Lexical Explanation:
«بَابُ الْحَثِّ» «حَثَّ يَحُثُّ حَثًّا»: To encourage, to embolden, to create enthusiasm and vigor, to incite.
«اَلْخُشُوع»: Outward and inward humility, meaning that every movement of all human limbs—eyes, heart, hands, feet, etc.—should be solely for Allah Ta'ala.
«مُخْتَصِرًا»: Is a shortened form of the active participle. Its explanation has been given by the author himself, that is, placing one's hands on the sides (flanks).
«خاصرة»: Refers to that part of the human body which is above the buttocks and below the ribs.

Benefits and Issues:
➊ Since prayer (salah) must be performed with complete attention and concentration purely for Allah, therefore, during it, no posture, movement, or action should occur that contradicts this quality of prayer. Standing with hands folded is itself an expression of respect.
➋ Standing with hands placed on the sides is an arrogant act, which is contrary to humility and meekness. In prayer, the posture and appearance should be of humility, submissiveness, and poverty, which is beloved to Allah.
➌ The state of arrogance and pride is disliked; therefore, "ikhtisar" (placing hands on the flanks) has been deemed undesirable in prayer. Moreover, this posture is that of the Jews, so it is also necessary to avoid resembling them.