Musnad Ahmad 30:629

'Abd al-Razzaq narrated to us, al-Muthanna ibn al-Sabah narrated to us, 'Amr ibn Shu'ayb informed me, from Sa'id ibn al-Musayyib, from Abu Hurayrah, who said: A Bedouin came to the Prophet, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, I am in the sands for four or five months, and among us are women in postpartum bleeding, menstruating women, and those in a state of major ritual impurity (junub). What is your opinion?" He said: "You must use the soil."