Bulugh al-Maram 101
Narrated Ibn Mas’ud (rad):
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”. [Reported by Al-Bukhari]
وَعَنِ ابْنِ مَسْعُودٍ ‏- رضى الله عنه ‏- قَالَ: { أَتَى اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-اَلْغَائِطَ, فَأَمَرَنِي أَنْ آتِيَهُ بِثَلَاثَةِ أَحْجَارٍ, فَوَجَدْتُ حَجَرَيْنِ, وَلَمْ أَجِدْ ثَالِثًا.‏ فَأَتَيْتُهُ بِرَوْثَةٍ.‏ فَأَخَذَهُمَا وَأَلْقَى اَلرَّوْثَةَ, وَقَالَ: 
"هَذَا رِكْسٌ" }  أَخْرَجَهُ اَلْبُخَارِيّ ُ  1‏ .‏‏1 ‏- صحيح.‏ رواه البخاري (156)‏.‏