Abu Dhar Radiyallahu 'anhu narrates that Rasullullah Sallallahu 'alaihi wasallam said: When anyone of you stands in Salat, he should not level the pebbles (with his hand), as the Mercy of Allah is directed towards him. (Tirmidhi: 379)
Note: In the early era of Islam, nothing was spread on the ground of
the masjid and Salat was performed upon pebbles. Therefore, performing
Sajdah would be uncomfortable due to the pebbles. Rasullullah Sallallahu
'alaihi wasallam had prohibited unnecessarily smoothening the place of
Sajdah, since this is the time when the Mercy of Allah is directed
towards the person in Salat. All such unnecessary actions during Salat
may become a means of depriving one of Allah's Mercy.