Sayyiduna Mu'ayqib (may Allah be pleased with him) reports that it was asked from the Messenger of Allah (peace and blessings be upon him) about touching pebbles in the mosque (during prayer), so he said: If you must do it, then you may do it once.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصلاة / 218