It is narrated from Sayyiduna Abu Dharr (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Clean earth is (a means of) ablution for the Muslim, even for ten years (if he does not find water, he should perform tayammum). When you find water, pour it over your body, for that is certainly better."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 15