Then he, (Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him), said: "If someone is in a rocky or stony area without any soil, he should place his hands on the stones and wipe his face and hands as described in the tayammum of soil. There is no harm in praying with this due to the situation of necessity, and there is no obligation to repeat the prayer." The reasoning behind this ruling is that stones are referred to as part of the earth (ard). Since this term applies to them, they fall under the general ruling previously mentioned. He, (Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him), also stated: "If the one performing tayammum finds water and is able to use it without fear for himself, then his prayer is not valid without performing ablution or ghusl with the water. However, there is no need to repeat the prayers performed with tayammum prior to finding water."
Commentary #10