Musannaf Ibn Abi Shayba 3:5283

Yahya ibn Sa'id narrated to us, from Ubaydullah, from Ibn al-Akhnas, who said: A freed slave of Abu Sa'id narrated to me, from Abu Sa'id, that he used to take three pebbles and place them on his thigh. He would perform tasbih (glorification of Allah), then place one [pebble down]. Then he would perform tasbih, place another, then perform tasbih, and place another. Then he would lift them and place [them down] in the same manner. And he said: "Do not perform tasbih with small [pebbles] for tasbih."