Purity: Performing Recommended Prayers with Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 706:
It is permitted for someone who cannot perform wuḍūʾ or ghusl to perform recommended prayers that have a specific time – such as the daily supererogatory (nāfilah) prayer – with tayammum. However, if one does not lose hope in his legitimate excuse expiring before the end of the time for such prayers, the obligatory precaution is that he must not perform them at the start of their time. Recommended prayers that do not have a specific time can be performed with tayammum at any time.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)