Hadith 500580
Chapter 2: Purity - كتاب الطهارة
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم سُئِلَ عَنْ الاِسْتِطَابَةِ فَقَالَ " أَوَلاَ يَجِدُ أَحَدُكُمْ ثَلاَثَةَ أَحْجَارٍ " .
Yahya related to me from Malik from Hisham ibn Urwa from his father that the Messenger of Allah, may Allah bless him and grant him peace, was asked about cleaning after excretion. He replied, "Are any of you unable to find three stones?"
Reference Muwatta Malik 500580
USC-MSA web (English) reference : Book 2, Hadith 28