Musannaf Ibn Abi Shayba 1:1671

Abu Mu'awiyah narrated to us, from al-A'mash, from Shaqiq, who said: I was sitting with Abdullah and Abu Musa. Abu Musa said: "O Abu Abd al-Rahman, what do you think if a man becomes in a state of major ritual impurity (junub) and does not find water for a month, how should he perform the prayer?" Abdullah said: "He should not perform tayammum, even if he does not find water for a month." Abu Musa said: "Then what about this verse in Surat al-Ma'idah: 'And you do not find water, then perform tayammum with pure soil'?" Abdullah said: "If they were granted a concession in this, they would soon, whenever the water became cold for them, perform tayammum with the soil."