Purity: Dust Retention in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 687:
Based on obligatory precaution, the thing with which one performs tayammum must have dust on it that will stay on the palms; and after striking his hands on it, he must not shake his hands vigorously, causing all the dust to fall off.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)