صحيح البخاري
Sahih Bukhari
كتاب التيمم
The Book of Tayammum (Rubbing Hands and Feet with Dust)
باب الصعيد الطيب وضوء المسلم، يكفيه من الماء:
Chapter. Clean earth is sufficient for a Muslim as a substitute for water for ablution (if he does not find water).