From Sahih Bukhari (Hadith 156):
Narrated `Abdullah:The Prophet (ﷺ) 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 I 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
Reference: Bukhari 156