Mishkat al-Masabih #488

‘Imran said: While we were on a journey with the Prophet he led the people in the prayer, and when he turned away after the prayer he saw a man apart who had not prayed along with the people. He asked him what had prevented him from praying along with the company, and when the man replied that he was affected by seminal defilement and had no water, he said, “Make use of earth, for it is enough for you.” (Bukhari and Muslim.)