Bukhari stated: ‘Faith is both speech (qawl) and actions (fi‘l).’
Ibn Rajab elaborates:
Most of the scholars have stated: Faith is [manifested through] both speech and actions. There is ijmâʿ(consensus) amongst the Salaf (pious predecessors) and the scholars of Hadith upon this. Al-Shâfiʿi and Abû Thawr maintained that there is ijmâʿ of the Ṣaḥâba (Companions of the Prophet) and Tâbiʿîn (Successors) upon this.
Al-Awzâʿi states: “Those amongst the earlier generations did not differentiate between faith and actions. Al-Fuḍayl ibn ʿIyâḍ and Wakîʿibn Al-Jarrâḥ have also narrated the same from Ahl Al-Sunnah wa Al-Jamâʿah.”
Amongst those about whom it is narrated that they proclaimed that “Faith is both speech and actions” are: Al-Ḥasan, Saʿîd ibn Jubayr, ʿUmar ibn ʿAbd Al-ʿAzîz, ʿAtâ’, Ṭâ’ûs, Mujâhid, Al-Shaʿbi, Al-Nakhaʿi, Al-Thawri, Al-Awzâʿi, Ibn Al-Mubârak, Mâlik, Al-Shâfiʿi, Aḥmad, Isḥâq, Abî ʿUbayd, Abû Thawr, among many others.
In fact, a lot of them even went as far as to say: “Indeed the freeing of a believing slave will not form a legally valid kaffâra (penitential expiation) until the affirmation [of the slave’s faith] is taken from him, i.e. until he shows his acceptance of, and abiding by, the commandments of obligatory prayer and fasting.” Amongst those who made such statements were: Al-Shaʿbi, Al-Nakha’i and Ahmad.
As for those who opposed the doctrine that faith is [manifested through] both speech and actions, they were some groups of scholars from the People of Kûfa and Baṣra, and they differentiated between ‘actions’ and ‘faith’, saying: “Faith (maʿrifa) is inner knowledge accompanied by qawl (verbal declaration).”
Other scholars from the generations subsequent to the foregoing scholars have stated: “Faith is only inner knowledge and nothing else,” whilst there were others who claimed: “Faith is only verbal declaration and nothing else.”
Bukhâri elaborates upon it [faith] saying: It is both qawl (verbal proclamation) and fi‘l (actions). As for the term fi‘l, there are those who say that it is simply another synonym for the term: ʿamal, and others who say that it is a more general term as compared to ʿamal. Amongst this latter group, there are those who elaborate on it, saying: “Fi‘l has in it, meanings of qawl (verbal declaration) and the action of the limbs, whilst ʿamal, does not necessarily have the meanings of qawl (verbal declaration) in it.” The words of ʿUbayd ibn ʿUmayr affirm this view: “Faith is not manifested by hoping (and wishing), but through “words said,” qawl yuqâl (قول يقال), and “actions done,” ʿamal yuʿmal (عمل يعمل).” Al-Khallâl narrated this.
Then there were others who said: ʿAmal is that which requires struggle and hard work, whilst fi‘l is something less specific (i.e. a more general term). Others have said: ʿAmal is that which causes an effect upon the thing being acted upon, as in the case that he who works with clay causes an effect upon the clay –and fi‘l does not have this specification in its meanings. There are yet others who have stated: ʿAmal is more honourable than fi‘l, as the word ʿamal has been used only for things which have connotations of honor and exaltation (achieved through struggle).
Bukhâri then states: ‘And it (faith) increases and decreases.’
That they would increase in faith along with their [present] faith. [Sûrat Al-Fatḥ, 48: 4]
And We increased them in guidance. [Sûrat Al-Kahf, 18:13]
And Allah increases those who were guided, in guidance. [Sûrah Maryam, 19: 76]
And those who are guided – He increases them in guidance and gives them their righteousness. [Sûrah Muḥammad, 47: 17]
And those who have believed will increase in faith. [Sûrat Al-Muddaththir, 74: 31]
Which of you has this increased in faith? As for those who believed, it has increased them in faith. [Sûrat Al-Tawbah, 9: 124]
A great army is gathering against you, so fear them.” But it [merely] increased them in faith. They said, “For us Allah is sufficient... [Sûrat Âl ʿImrân, 3: 173]
And it only increased them in faith and acceptance. [Sûrat Al-Aḥzâb, 33: 22]
According to the majority of scholars, faith increases and decreases, i.e. it fluctuates. This view has also been recorded as the view of a large group of the Companions, e.g. Abû Al-Dardâ’ and Abû Hurayrah, Ibn ʿAbbâs,  and other such prominent Companions of the Prophet ﷺ. Also, words denoting the same meaning have been transmitted by ʿAli, Ibn Masʿûd, and from Mujâhid, and other Tabiʿîn too.
There were some who refrained from saying that faith decreases, saying that “it increases,” but not saying that “it decreases.”  This was said to be the view of Mâlik. (It is well-known that [Mâlik] concurs with the view of the majority as regards the increase and decrease of faith.) 
It is narrated from Ibn Al-Mubârak, who said: “The faith [of believers] can excel [one over the other],”  and this means that faith increases and decreases: it fluctuates.
Bukhari went on to quote Qur’anic verses in which the increasing of faith is mentioned. Prominent persons from amongst the Salaf, like ʿAtâ’ ibn Abi Rabâḥ and those who came after him, have also argued from these verses that faith both increases and decreases.
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Bukhâri has likewise quoted Qur’anic verses in which the increase of guidance has been mentioned, and what is meant here by ‘guidance’ is: The doing of good deeds –in that Allah describes the muttaqîn (God-fearing people) as those who have faith in the Unseen, who uphold Prayer, give charity from that which Allah has given them, believe in that which has been revealed to Muhammad ﷺ and those who came before him (amongst Allah’s Messengers), and believe in the Hereafter, and who then says:
Those are upon [right] guidance from their Lord. [Sûrat Al-Baqarah, 2: 5]
And so in this way, Allah refers to all of the acts mentioned in the âyah above as ‘guidance (huda),’ and hence he whose obedience to Allah increases, his guidance simultaneously increases.
Thus, because faith comprises inner knowledge that comes from the heart, verbal declaration, and physical actions, then all of it increases with an increase in actions (Al-ʿamal), and decreases with the decrease in actions. A lot of the Salaf have affirmed this in clear terms, saying: “[Faith] increases through acts of obedience, and it decreases through the committing of sins.”
As for the rising of faith due to an increase in verbal declarations, and its downfall due to a decrease in verbal declarations, its case is similar to the case of the actions of the limbs. Hence, he who increases the remembrance of Allah and the recitation of His book, then his faith will also increase. As for him who abandons the obligatory remembrances which are required to be uttered through the tongue, then his faith also decreases.
As for having inner knowledge/belief (maʿrifa) in the heart: Does that rise and fall too? There are two opinions regarding this.
- The first one is that: it does not rise nor does it fall.Yaʿqûb ibn Bakhtân  said: I asked Abû ʿAbdullâh — i.e. Aḥmad ibn Ḥanbal– about maʿrifa (inner knowledge) and verbal declarations: “Do they increase and decrease?” He replied: “No, we came forth with qawl (verbal declarations), and maʿrifa (inner knowledge), and what remained was ʿamal (actions).”Al-Khallâl remarked in his book ‘Al-Sunnah’:  “What Imam Ahmed intends by the word qawl is the utterance of the Muslim profession of faith (Al-Shahâdatayn).”
- The Second opinion is that: maʿrifa increases and decreases.Al-Mârwadi said, “I asked Imam Ahmed about the inner knowledge of Allah: Does it differ from one person to another, or from one time or state to another [i.e. within the heart of the same person]?” Imam Ahmed replied: “Yes.” I further asked: “Does it also increase?” And he said, “Yes.”
To be continued…
 Look up most of these sayings in ‘Al-Sharîʿah’ by Al-Âjurri (p.120-132).
 Translator’s note: Both Arabic words, Fi‘l and ʿamal may be translated as ‘actions’ or ‘deeds’.
 In the book ‘Al-Sunnah’, (1212), and in it (the narration) is Ibn Lahîʿah.
 ‘Al-Sharîʿah’ (p. 111).
 See ‘Al-Sunnah’, by Al-Khallâl (p. 976-979).
 ‘Al-Sunnah’, by Al-Khallâl: (1014), and ‘Al-Sharîʿah’, (p. 118).
 ‘Al-Sunnah’, by Al-Khallâl: (1018).
 He is Yaʿqûb ibn Isḥâq ibn Bakhtûn, and there is a biographical notice of him in Ṭabaqât Al-Hanâbilah (1/415).