It is narrated from Sayyiduna Mu'ayqib (may Allah be pleased with him) that he asked the Prophet (peace and blessings be upon him) about levelling (the pebbles) during the prayer, and he (peace and blessings be upon him) said: "Do it once, or else leave it."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17952