Sahih Bukhari Volume 001, Book 007, Hadith Number 334.

Narated By 'Abdur Rahman bin Abza : A man came to 'Umar bin Al-Khattab and said, "I became Junub but no water was available." 'Ammar bin Yasir said to 'Umar, "Do you remember that you and I (became Junub while both of us) were together on a journey and you didn't pray but I rolled myself on the ground and prayed? I informed the Prophet about it and he said, 'It would have been sufficient for you to do like this.' The Prophet then stroked lightly the earth with his hands and then blew off the dust and passed his hands over his face and hands."