Bukhari

Hadith Commentary:

Because doing so repeatedly is contrary to humility and submissiveness (khushu' and khudu') in prayer.

Hadith Commentary:
(1)
In one hadith, the Messenger of Allah (sallallahu alayhi wa sallam) said:
“When any one of you is performing prayer, he should not remove pebbles from his place of prostration, because at that time the mercy of Allah is before him.”
(Sunan Abi Dawud, Kitab al-Salat, Hadith: 945)
However, if the pebbles cause discomfort, they may be removed once; repeating this action again and again is not correct.
In another narration, the wording is:
“If it is necessary to remove the pebbles, then remove them once, or leave them as they are.”
(Musnad Ahmad: 5/163)
Although some contemporaries have declared this hadith weak, it is nevertheless valid as evidence because it is supported by the aforementioned hadith from Sahih al-Bukhari.
The reason for the prohibition of repeatedly removing pebbles in the hadith is that during prayer, the mercy of Allah is before the worshipper; thus, diverting attention to repeatedly smooth out the pebbles is akin to turning away from the mercy of Allah.

(2)
The title mentions pebbles, whereas the hadith uses the word turab, meaning soil. Since there are generally pebbles in Mina, the title is established from this hadith on the basis of predominant assumption.
It is possible that Imam al-Bukhari (rahimahullah) intended to indicate that the ruling is the same for both pebbles and Mina. Furthermore, in the narration of Sahih Muslim, the word “pebbles” is also mentioned.
(Sahih Muslim, Kitab al-Masajid, Hadith: 1219(546))

Hadith Commentary:
Benefits and Issues:
In prayer, repeatedly cleaning the place of prostration while performing prostration is not correct; this action is contrary to the etiquettes and humility of prayer. In case of necessity, it is permissible to do so only once.

946. Commentary:
According to Shaykh al-Albani rahimahullah, this narration is weak, but based on supporting evidences, it is suitable for use as proof. Therefore, a person performing prayer should clean his place and arrange the prayer mat, etc., properly before beginning the prayer. Doing this action during the prayer is not permissible; however, if it must be done, then there is concession for it only once.

Benefits and Issues:
➊
If during the prayer it is felt that the pebbles are too uneven,
which are pricking the face and causing distraction from the prayer,
then it is permissible to smooth them out slightly with the hand once. Excessive effort in this regard is not appropriate.

➋
The prayer is not invalidated by movements that are contrary to humility (khushu‘) in prayer, but the reward is diminished.
Therefore, due to excessive movements, the reward can be greatly reduced,
which is an extreme loss for the believer.