Musannaf Ibn Abi Shayba 3:5449

Waki' narrated to us, he said: Hisham al-Dastawa'i narrated to us, from Yahya ibn Abi Kathir, from Abu Salamah ibn 'Abd al-Rahman, from Mu'ayqib, he said: The wiping of pebbles was mentioned to the Prophet, peace and blessings of Allah be upon him, and he said: "If you must do it, then [do it] once."