Musnad Ahmad 937:278

Abu Ahmad narrated to us, Sufyan narrated to us, from Khalid al-Hadhdha’, from Abu Qilabah, from ‘Amr ibn Bujdan, from Abu Dharr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the pure soil is the ablution of the Muslim, even if he does not find water for ten years. Then, if he finds it, let him touch his skin with it, for that is better."