Al-Iman: Words and actions which increase with obedience and decrease with disobedience.
إِنَّمَا الْمُؤْمِنُونَ الَّذِينَ إِذَا ذُكِرَ اللَّهُ وَجِلَتْ قُلُوبُهُمْ وَإِذَا تُلِيَتْ عَلَيْهِمْ آيَاتُهُ زَادَتْهُمْ إِيمَانًا وَعَلَىٰ رَبِّهِمْ يَتَوَكَّلُونَ * الَّذِينَ يُقِيمُونَ الصَّلَاةَ وَمِمَّا رَزَقْنَاهُمْ يُنفِقُونَ
Meaning: “The believers (people of Iman) are only those who, when Allah is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely -The ones who establish prayer, and from what We have provided them, they spend.”
وَمَا كَانَ اللَّهُ لِيُضِيعَ إِيمَانَكُمْ
Meaning: “And never would Allah have caused you to lose your Iman (prayers before the change of Qibla.”
So here prayer which is actions and words has been called as Iman.
From Iman, there is:
(1) Iman Asli: Nullification of this means the one is completely outside the fold of Islam. This can occur by Shirk, denial of something known by necessity in the Shariah, allegiance to the disbelievers against the Muslims or loving them for their Deen, leaving prayers etc. One of them is enough for the negation of Iman Asli and the one who negates it has negated his Iman completely and is a Kafir whether he says the Shahadath or not.
(2) Iman Wajib: This is leaving something which is Wajib or doing something which is Haram. If Allah wills He will punish the person, if He wills, He will forgive him.
(3) Iman Mustahabb: This is leaving something which is Mustahabb (an order in the Shariah but not in the meaning of compulsion) or doing something which is Makruh (forbidden in the Shariah but not in the meaning of compulsion of leaving it.) Allah will not punish someone over this.
From what increases Iman is knowledge and acting upon it. Allah said:
أَمَّنْ هُوَ قَانِتٌ آنَاءَ اللَّيْلِ سَاجِدًا وَقَائِمًا يَحْذَرُ الْآخِرَةَ وَيَرْجُو رَحْمَةَ رَبِّهِ ۗ قُلْ هَلْ يَسْتَوِي الَّذِينَ يَعْلَمُونَ وَالَّذِينَ لَا يَعْلَمُونَ
Meaning: “Is one who is devoutly obedient during periods of the night, prostrating and standing [in prayer], fearing the Hereafter and hoping for the mercy of his Lord, [like one who does not]? Say, “Are those who know equal to those who do not know?””
So in the Ayat there is Dalil that knowledge and action upon it increases Iman.