Sahih Bukhari Volume 001, Book 004, Hadith Number 158.
Narated By 'Abdullah : The Prophet went out to answer the call of nature and asked me to bring three stones. I found two stones and searched for the third but could not find it. So took a dried piece of dung and brought it to him. He took the two stones and threw away the dung and said, "This is a filthy thing."