It is narrated from Sayyiduna Mu'ayqib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked about straightening pebbles in the mosque (while prostrating), so he (peace and blessings be upon him) said: "If you must do it, then straighten them only once."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المباحة فى الصلاة / 895
Sayyiduna Jabir (may Allah be pleased with him) narrates that I asked the Messenger of Allah (peace and blessings be upon him) about straightening pebbles during the prayer, so he (peace be upon him) said: "Do it once. And if you do not straighten them, then it is better for you than a hundred she-camels, all with black eyes (i.e., young and healthy)."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المباحة فى الصلاة / 897