Sahih Muslim
Sahih Muslim 546 a
It is disliked to smooth the pebbles or make the dirt level during salat
Arabic hadith text
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا هِشَامٌ الدَّسْتَوَائِيُّ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي سَلَمَةَ، عَنْ مُعَيْقِيبٍ، قَالَ ذَكَرَ النَّبِيُّ صلى الله عليه وسلم الْمَسْحَ فِي الْمَسْجِدِ - يَعْنِي الْحَصَى - قَالَ "إِنْ كُنْتَ لاَ بُدَّ فَاعِلاً فَوَاحِدَةً "
Translation
Mu'aiqib quoted the Messenger of Allah (ﷺ) mentioning the removal of pebbles from the ground where he prostrated himself. He (the Prophet) said
It you must do so, do it only once.