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."  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا قَامَ أَحَدُكُمْ إِلَى الصَّلاَةِ فَلاَ يَمْسَحِ الْحَصَى فَإِنَّ الرَّحْمَةَ تُوَاجِهُهُ قَالَ وَفِي الْبَابِ عَنْ مُعَيْقِيبٍ وَعَلِيِّ بْنِ أَبِي طَالِبٍ وَحُذَيْفَةَ وَجَابِرِ بْنِ عَبْدِ اللَّهِ  

"I asked Messenger of Allah (S) about smoothening the pebbles during Salat. He said: 'If you must do so, then let it be only once.'"  

سَأَلْتُ رَسُولَ اللَّهِ ﷺ عَنْ مَسْحِ الْحَصَى فِي الصَّلاَةِ فَقَالَ إِنْ كُنْتَ لاَ بُدَّ فَاعِلاً فَمَرَّةً وَاحِدَةً