Musannaf Ibn Abi Shayba 3:5284

Ibn ‘Ulayyah narrated to us, from al-Jurayri, from Abu Nadrah, from a man from the Dhifar [tribe], who said: I stayed as a guest with Abu Hurayrah, and he had with him a pouch containing pebbles or date pits. He would say, "Subhan Allah, Subhan Allah," until, when what was in the pouch was exhausted, he would throw it to a black slave girl, and she would gather them, then return them to him.