Musnad Abu Ya'la al-Mawsili 105:5

Al-Qawariri narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Salamah ibn Kuhayl, from Abu Malik and Abdullah ibn Abd al-Rahman ibn Abza, from his father, Abd al-Rahman ibn Abza, who said: We were with Umar when a man came to him and said, "O Commander of the Faithful, we stay for a month or two and do not find water." Umar said, "As for me, I would not pray until I found water." Ammar said, "O Commander of the Faithful, do you remember when we were in such-and-such a place, and we were tending the camels, and we discussed that we had become ritually impure (junub)?" He said, "Yes, I rolled in the dust, then I came to the Prophet, peace and blessings of Allah be upon him, and told him. He laughed and said, 'The earth would have been sufficient for you,' and he struck the ground with his palms, then blew into them, then wiped his face and parts of his forearms." He (Umar) said, "Fear Allah, O Ammar." He said, "O Commander of the Faithful, if you wish, I will never mention it as long as I live." He said, "No, but we shall entrust you with that which you have taken charge of."