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