It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that: "When you are a traveller and you are junub (in a state of major ritual impurity), or you are without ablution, and you fear that if you perform ablution with the water you will die of thirst, then do not perform ablution; rather, save your life."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1109