Musnad Ahmad 648:18

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sulayman, from Abu Wa'il, who said: Abu Musa said to Abdullah ibn Mas'ud: "If we do not find water, do we not pray?" Abdullah said: "Yes. If we do not find water for a month, we do not pray. And if I were to grant them a concession in this, then whenever one of them felt cold, he would say: 'Like this'—meaning he would perform tayammum and pray." I said to him: "Then what about the statement of Ammar to Umar?" He said: "I did not see Umar convinced by the statement of Ammar."