Musannaf Ibn Abi Shayba 1:1639

Hushaym narrated to us, he said: Abu Bishr informed us, from Tawus, who said: "Istinja (cleaning oneself after defecation) is with three stones." He said: I said, "If I do not find three stones?" He said, "Then three sticks." I said, "If I do not find three sticks?" He said, "Then three handfuls of soil."