Musannaf Ibn Abi Shaybah

Hazrat Abu Saeed’s practice was that he would take three pebbles and place them on his thigh. Then he would recite the tasbih once and pick up one pebble, then recite the tasbih and pick up another pebble, then recite the tasbih and pick up the third pebble as well. Then he would put all the pebbles back and repeat this action. And he would say: Do not recite the tasbih in such a way that it sounds like a whistle.