Purity: Direction and Continuity in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 693:
Based on obligatory precaution, the forehead and the back of the hands must be wiped from top to bottom [i.e. the forehead must be wiped in a direction towards the eyebrows and above the nose, and the back of the hands must be wiped in a direction towards the fingertips]. These actions must be performed one after the other; if there is a delay between performing them such that it cannot be said that one is performing tayammum, then the tayammum is invalid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)