Purity: Performing Tayammum Before Prayer Time
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 703:
If someone whose duty is to perform tayammum loses hope in his legitimate excuse [for performing tayammum in place of wuḍūʾ or ghusl] expiring during the entire length of time for prayers, or if he deems it probable that if he delays performing tayammum he will not be able to perform tayammum in time, he can perform tayammum before the time for prayers has set in. If one performs tayammum for another obligatory or recommended act and his legitimate excuse remains valid until the time of prayer has set in, he can perform the prayer with that tayammum.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)