Adab as-Salat: The Disciplines of the Prayer Second Revised Edition
Final Note by Imam Khomeini
Know that for the salāt, beside its form, there is a meaning, and apart
from its exterior it has an interior; and as the exterior has its
disciplines, neglecting which would render the outer form of the salāt
invalid or incomplete; likewise, its interior has cordial spiritual
disciplines, neglecting which would render the spiritual salāt invalid
or incomplete; whereas observing them would inspire the salāt with a
heavenly spirit.
Imām Khomeinī.