It is permissible to turn the pebbles for prostration once; due to the impossibility of prostrating, so he levels it once, and it is disliked to do so more than once; as narrated by Mu'ayqib, may Allah be pleased with him, who said: The Messenger of Allah, peace be upon him, said: "Regarding a man who levels the dirt where he prostrates, he said: If you must do it, then do it once." This is found in Sahih al-Bukhari 1: 404, and Sahih Ibn Khuzaymah 2: 51. Refer to: Sharh al-Wiqayah, pp. 164-166, and Tabyin al-Haqaiq 1: 162-167.