Sayyiduna Ali (may Allah be pleased with him) says regarding a person who is traveling, becomes junub, and does not have water with him, due to which he fears remaining thirsty, Sayyiduna Ali (may Allah be pleased with him) says: "That person should perform tayammum, he should not perform ghusl."
Hadith Referenceسنن الدارقطني / كتاب الطهارة / 774
Hadith Takhrij«أخرجه الدارقطني فى ((سننه)) برقم: 774»