Sahih Bukhari Volume 001, Book 007, Hadith Number 335.

Narated By Said bin 'Abdur Rahman bin Abza : (On the authority of his father who said) 'Ammar said so (the above Statement). And Shu'ba stroked lightly the earth with his hands and brought them close to his mouth (blew off the dust) and passed them over his face and then the backs of his hands. 'Ammar said, "Ablution (meaning Tayammum here) is sufficient for a Muslim if water is not available."