Shafaqna Pakistan: The Grand Ayatollah Sistani answered a question about performing Salaat by doing Tayammom.
Question: Is it possible to perform Salaat/Salaah promptly (at exactly specified time for Salaat) by doing Tayammom?
The Grand Ayatollah Sistani: It is allowed if (the person) knows that his/her excuse remains to the end of specific time (for performing Salaat); otherwise must wait.
Source: Shafaqna English