Musannaf Ibn Abi Shayba 3:5447

Abu Bakr narrated to us, he said: Abdullah bin Numayr narrated to us, from Ibn Abi Layla, from Isa bin Abd al-Rahman bin Abi Layla, from his father, from Abu Dharr, who said: I asked the Messenger of Allah (peace and blessings of Allah be upon him) about things, until I asked him about touching the pebbles [during prayer]. He said: "Once, and if not, then leave it."