Hadith: When anyone of you stands in prayer, indeed mercy is facing him. So, let him not smooth the pebbles

عن أبي ذر رضي الله عنه مرفوعًا: «إذا قام أَحَدُكُمْ إلى الصلاة؛ فإنَّ الرَّحمة تُوَاجِهُهُ، فلا يَمْسَح الحَصَى». وعن معيقيب رضي الله عنه أن النبي صلى الله عليه وسلم قال: في الرجل يُسَوِّي التُّراب حيث يسجُد، قال: «إِنْ كُنْت فاعِلا فَوَاحِدة».
[حديث أبي ذر ضعيف، وحديث معيقيب صحيح] - [حديث أبي ذر رواه أبو داود. حديث معيقيب متفق عليه]

Abu Dharr (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "When anyone of you stands in prayer, indeed mercy is facing him. So, let him not smooth the pebbles." Mu‘ayqīb (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said about the man who levels the earth where he prostrates: "If you must do so, then do it only once."
Authentic hadith - Narrated by Bukhari

Explanation

In the Hadīth of Abu Dharr, the statement: "When anyone of stands in prayer," means when one starts to pray, and thus it is not forbidden to remove the pebbles before starting the prayer. Removing the pebbles is forbidden in this Hadīth so that insignificant things should not distract the Muslim from the prayer or interrupt his devotion to it. In fact, doing so causes the Muslim to miss out the mercy resulting from focusing on the prayer. This applies to the case in which the praying person does not need to fix the place of prostration; otherwise, it is permissible to do it one time only as required. Pebbles in specific are mentioned in the Hadīth because they were commonly found on the ground of their mosques back then. Otherwise, there is no difference between pebbles, dust, and sand in this regard. The Hadīth of Mu‘ayqīb states that the Prophet (may Allah's peace and blessings be upon him) said about a man who was leveling the earth where he is prostrating: "If you must do so, then do it only once." A man asked the Prophet (may Allah's peace and blessings be upon him) about his case in which he used to level the earth where he prostrates or for prostrating on it. As this was done during the prayer, the Prophet (may Allah's peace and blessings be upon him) instructed the man to level the earth only one time if there is a need for it. Smoothing the pebble in prayer is disliked except in the case where the pebbles do not allow the person to do prostration properly because they are too uneven to place the required part of the forehead on them. In this case, therefore, one may smooth the pebbles once or twice. In fact, there are two narrations reported on this issue. One narration says that removal of the pebble should be done once, and the other narration says it should be twice. The more apparent meaning of the two narrations implies that one may wipe the pebbles once and not more. The cause for the prohibition of smoothing the pebbles during the prayer is that mercy descends upon and showers the praying person. So, it is not appropriate for a sensible person to react to this significant blessing by such an insignificant action, as stated by At-Tībi. Ash-Shawkāni also said: "This justification highlights that the wisdom behind the prohibition of smoothing the pebbles is that one should not busy his mind with something, which distracts him from the mercy facing him, and thus missing out his share of it."