It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Tayammum is two strikes: one strike for the face, and one strike for the two hands up to the elbows."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13309