Musnad Ahmad 30:1505

Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, from al-Muthanna ibn al-Sabah, from Amr ibn Shu’ayb, 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: "We are in this sand and we do not find water, and among us are the menstruating woman, the one in a state of major ritual impurity (junub), and the woman experiencing postpartum bleeding, and four months may pass upon her without finding water." He said: "Upon you is the dust," meaning tayammum.