Sunan An-Nasai, Chapter: The Book of Purification

"We were with Umar when a man came to him and said: 'O Commander of the Believers! Sometimes we stay for a month or two without finding any water. Umar said: As if I did not find water, I would not pray until I found water.' 'Ammar bin Yasir said: 'Do you remember, O Commander of the Believers, when you were in such and such a place and we were rearing the camels, and you know that we became Junub?' He said: 'Yes.' As for me I rolled in the dust, then we came to the Prophet (saw) and he laughed and said: "Clean earth would have been sufficient for you." And he struck his hands on the earth then blew on them, then he wiped his face and part of his forearms. He (Umar) said: "Fear Allah, O 'Ammar!'" He said: 'O Commander of the Believers! If you wish I will not mention it.' He said: 'No, we will let you bear the burden of what you took upon yourself.'" (Sahih)