Musannaf Ibn Abi Shayba 35:244

Waki’ narrated to us, from al-A’mash, from Salamah ibn Kuhayl, from Ibn Abza, from his father, who said: Umar said to Ammar: "Do you not remember the day we were in such-and-such a place and became ritually impure (junub), and we did not find water, so we rolled in the dust? When we came to the Prophet, may Allah bless him and grant him peace, we mentioned that to him, and he said: 'This would have sufficed you both.'" And al-A’mash struck with his hands a single strike, then blew into them, then wiped his face and his palms with them. And it was mentioned that Abu Hanifah said: "Two strikes; one strike does not suffice him."