Sahih Bukhari Volume 1, Book 7, Hadith Number 344

Narated By ‘Imran bin Husain Al-Khuza’i : Allah’s Apostle saw a person sitting aloof and not praying with the people. He asked him, “O so and so! What prevented you from offering the prayer with the people?” He replied, “O Allah’s Apostle! I am Junub and there is no water.” The Prophet said, “Perform Tayammum with clean earth and that will be sufficient for you.”