Abu Qilabah narrates from a man of the tribe of Banu Amir, who said: I saw Sayyiduna Abu Dharr (may Allah be pleased with him), and he narrated the hadith about janabah, in which it is mentioned that he became junub (in a state of major impurity) and (he had) no water. The Messenger of Allah (peace and blessings be upon him) said: "O Abu Dharr! Clean earth is sufficient for you, even if you do not find water for ten years; and when you find water, let it touch your skin (that is, take a ritual bath)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 847