the Prophet (S) said: "When one of you stands for Salat then he should not smoothen the pebbles, for indeed it is mercy that he is facing."
Jami` at-Tirmidhi Book 2, Hadith 231
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Jami` at-Tirmidhi Book 2, Hadith 230 Next Hadith → "I asked Allah's Messenger (S) about smoothening the pebbles during Salat. He said: 'If you must do so, then let it be only once.'"

Jami` at-Tirmidhi Book 2, Hadith 232