I say, and with Allah's success: If dust touches his face and hands, it suffices for him to perform tayammum, but two strikes are enough: one strike for the face, and one strike for the hands up to the elbows. It is a condition that the wiping is not done with less than three fingers of the hand, as it is necessary to wipe with more than that, and the minimum is three fingers. It was narrated by Abdul Rahman bin Abzi : "A man came to Umar  and said: I have become impure and have not found water? He said: Do not pray. Then Ammar said: Do you not remember, O Commander of the Faithful, when you and I were in a military expedition and we both became impure and did not find water, you did not pray, but I rolled in the dust and prayed. The Prophet  said: It would have sufficed you to strike your hands on the ground, then blow, and then wipe your face and hands with them," in Sahih Muslim 1: 280, and Sahih Bukhari 1: 129. See: Al-Hadiyah Al-Ala'iyah p. 35, and Allah knows best.