All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
It can be deduced from this verse that the action of the heart is distinct from the statement of the heart; the action of the heart includes Khushoo' (feeling humble) - which is mentioned in the verse - and Khushoo' consists of submission and humility to Allaah and similar devotional actions of the heart. The statement of the heart, on the other hand, refers to the conviction in the heart and faith therein, which is also mentioned in the verse. Faith is completed with these two (action of the heart and statement of the heart) along with the statement of the tongue and the action of the body parts.
The Maaliki scholar Ibn Abi Zayd said in Ar-Risaalah, “Faith is made up of three components: (a) statement of the tongue, (b) conviction of the heart, (c) and actions of the body. The statement of faith cannot be completed except with action, and the statement and action cannot be completed except with intention.” [Ar-Risaalah]
Ibn Katheer said in his Tafseer, “The essence of faith is Tasdeeq (affirmation); it comprises the statement of the heart (namely the affirmation), the action of the heart (namely the intention and sincerity), the action of the body, and the statement of the tongue...” [Tafseer Ibn Katheer]
As-Saʻdi said in his Tafseer, “Faith is the statement of the heart and tongue, the action of the heart, tongue, and body; and the believers vary greatly in these matters.”
Hence, we can say that the verse indicates the components and divisions of faith and the distinction between them. This has been clearly addressed in many verses. For instance, Allaah, The Exalted, says (what means): {Except for those who have believed and done righteous deeds...} [Quran 103:3] As-Saʻdi said in his Tafseer, “The verse means those who believed in their hearts and acted upon it with the body parts. This description encompasses all religion: its tenets, actions, fundamentals (Usool), and branches (Furoo'), regardless of whether that be outward or inward.”
Allaah knows best.