The Prophet (ﷺ) said: When one of you gets up to pray, he must not remove pebbles, for mercy is facing him.
Sunan Abi Dawud Book 2, Hadith 556
← Previous Hadith The Prophet (ﷺ) said: Saying Tasbih applies to men during prayer and clapping applies to women. Anyone who makes a sign during his prayer, a sign which is intelligible by implication, should repeat it (i.e. his prayer).

(AbuDawud commented on the Hadith saying, this is a result of confusion.)

Sunan Abi Dawud Book 2, Hadith 555 Next Hadith → Mu’aiqib reported the Prophet (ﷺ) as saying ; Do not remove pebbles while you are praying; if you do it out of sheer necessity, do it only once to smooth the pebbles.

Sunan Abi Dawud Book 2, Hadith 557