It is narrated from Sayyiduna Ali (may Allah be pleased with him) that this verse, "Nor when you are junub, except when passing through (a way), until you have bathed" [Surah al-Nisa: 43], was revealed concerning the traveller, and he said: "When a person becomes junub and does not find water, let him perform tayammum and pray. When he finds water, let him take a ritual bath (ghusl)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1037