Sayyiduna Ali (may Allah be pleased with him) said: "When you become junub (in a state of major ritual impurity) and you intend to perform ablution or ritual bath (ghusl), but you have no water except what you drink, and you fear for your life, then perform tayammum."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1108