Purity: Extent of Wiping the Back of the Hand
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 692:
If someone is not certain that he has wiped all of the back of his hand, then, to be certain, he must wipe an area a little higher than his wrist as well. However, it is not necessary to wipe in between the fingers.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)