It is narrated from Sayyiduna Abu Dharr (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Clean earth is the means of ablution for the Muslim, even if he does so for ten years; and when he finds water, let him pour it over his body, for that is better."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 870