Purity: Completeness of Wiping in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 691:
If a person fails to wipe even a small area of his forehead or the back of his hands, his tayammum is invalid, irrespective of whether he fails to wipe the area intentionally, or because he did not know the ruling or had forgotten to wipe it. However, it is not necessary to be very particular either, and it is sufficient if it can be said that the entire forehead and back of the hands have been wiped.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)