Sayyiduna Ammar bin Yasir (may Allah be pleased with him) narrates from the Messenger of Allah (peace be upon him) regarding tayammum that he (peace be upon him) said: "For the face and hands, there is only one strike." (One should strike the earth with the hands only once.)
Hadith Referenceصحيح ابن خزيمه / جماع أبواب التيمم عند الإعواز من الماء فى السفر، / 267