Explanation of the Book of Prayer-12

The Shaykh :" And from Abu Dharr (may Allah be pleased with him), he said: The Messenger of Allah (peace and blessings be upon him) said:

When any one of you stands in prayer, let him not smooth the pebbles, for he is facing Mercy.

إذا قام أحدكم في الصلاة فلا يمسح الحصى فإن الرحمة تواجهه

Sunan an-Nasa'i 1191Hasan (Darussalam)

The five [books] narrated it with a sound chain, and Ahmad added:

One [pebble], or leave it.

واحدة أو دع

".
By the pebbles, what is meant is those with which the mosque was paved. In the time of the Prophet (peace and blessings be upon him), mosques were paved with pebbles, small pebbles.
And his saying means that if he prostrates on the pebbles, despite their hardness and firmness, then mercy is facing him, meaning that this becomes a cause for mercy. This is because of the difficulty of prostrating on them, for there is a difference between prostrating on pebbles and prostrating on one's bed. We say regarding the pebbles: do not brush them away; leave them as they are and prostrate on them so that you may attain mercy thereby, since you have performed prostration to Allah (Mighty and Majestic) despite its difficulty.