Sahih Bukhari Volume 001, Book 004, Hadith Number 157.

Narated By Abu Huraira : I followed the Prophet while he was going out to answer the call of nature. He used not to look this way or that. So, when I approached near him he said to me, "Fetch for me some stones for ' cleaning the privates parts (or said something similar), and do not bring a bone or a piece of dung." So I brought the stones in the corner of my garment and placed them by his side and I then went away from him. When he finished (from answering the call of nature) he used, them.