Musnad Ahmad 33:92

Waki’ narrated to us, from Ibn Abi Dhi’b, from Sharhabil ibn Sa’d, 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 if you refrain from it, it is better for you than a hundred camels, all of which have black pupils."