Sayyiduna Abu Dharr (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When any one of you stands in prayer, the mercy of the Lord turns towards him, so do not move the pebbles." (Do not turn your attention to things other than the prayer.)
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المكروهة فى الصلاة التى قد نهي عنها المصلي / 914