(a) It is narrated from Sayyiduna Abu Dharr (may Allah be pleased with him): Moving the pebbles is permissible only once, and if I do not do it even once, that is dearer to me than a hundred she-camels with black pupils.
(b) It is narrated from Mujahid that Sayyiduna Abu Dharr (may Allah be pleased with him) reports from the Prophet (peace and blessings be upon him) concerning moving the pebbles that he (peace and blessings be upon him) said: "Moving them only once is permissible."
(c) It has been narrated to us from Sayyiduna Uthman bin Affan (may Allah be pleased with him) that he used to level the pebbles with his shoes before starting the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3549