Musannaf Ibn Abi Shayba 3:5450

Waki’ narrated to us, he said: Ibn Abi Dhi’b narrated to us, from Sharhabil Abu Sa’id, from Jabir ibn ‘Abdullah, who said: I asked the Prophet, peace and blessings of Allah be upon him, about smoothing the pebbles during prayer. He said: "One [time]. And that you refrain from it is better for you than a hundred she-camels, all of which have black pupils."