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.