Purity: Priority of Procuring Soil for Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 676:
If a person can gather soil by shaking a carpet and similar things, then performing tayammum with something that is merely dusty is invalid. Similarly, if one can dry some mud and procure soil from it, then performing tayammum with mud [that has not been dried] is invalid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)