Musnad Ahmad 937:83

'Abd al-Razzaq narrated to us, Sufyan informed us, from Ayyub al-Sakhtiyani and Khalid al-Hadhdha', from Abu Qilabah—both of them mentioned it: Khalid from 'Amr ibn Bujdan, and Ayyub from a man, from Abu Dharr: That Abu Dharr came to the Prophet, may Allah bless him and grant him peace, while he was in a state of major ritual impurity (janabah). The Prophet, may Allah bless him and grant him peace, called for water for him, so he screened himself and performed ghusl. Then he said to him: "Indeed, the pure earth is an ablution for the Muslim, even if he does not find water for ten years. But when he finds water, let him touch it to his skin, for that is better."