"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
← Previous Hadith 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 Next Hadith → "The Prophet (S) saw a boy of ours - called Aflah - blowing when he prostrated. So he said: 'O Aflah! Put your face in the dirt.'"

Jami` at-Tirmidhi Book 2, Hadith 233