When the hypocrites come to you (O Muhammad SAW), they say: "We bear witness that you are indeed the Messenger of Allâh." Allâh knows that you are indeed His Messenger and Allâh bears witness that the hypocrites are liars indeed. [Munaafiqoon 63:1]

Here the hypocrites are treated as liars, even though they expressed with their tongues their testification that Muhammad is indeed the Messenger of Allaah. When someone testifies to something then there must be two conditions present: a) The hearts firm knowledge of, belief, and certainty in that for which it is testifying, b) the tongue’s expression of that which the heart is certain of.

So the hypocrites were liars because their tongues uttered something different to that which their hearts contain. Another proof for ‘the Speech of the Heart’ is the saying of Allaah,

They were that day, nearer to disbelief than to Faith, saying with their mouths what was not in their hearts. And Allâh has full knowledge of what they conceal [Aali Imraan 3:167] See also al-Fath 48:11

So what we understand as ‘Speech of the Heart’ is explained in the Qur’aan

The Speech of the Tongue

Both of the above two verses are also an indication of the requirement of the ‘Speech of the Tongue’.

The Actions of the Heart, Tongue and Limbs

The Qur’aan is overwhelming with descriptions of the acts of worship, whether related to the Heart, Tongue or Limbs and the purpose here is to be brief and not prolong the discussion. See for yourself how Allaah describes, the love, fear and hope of the believers, their enjoining the good and forbidding the evil, their remembrance of Him and seeking forgiveness from Him and also their physical exertion in worshipping Him, such as constancy in prayer, fasting, etc.

Eemaan Increases and Decreases

From the Qur’aan:

The believers are only those who, when Allâh is mentioned, feel a fear in their hearts and when His Verses (this Qur'ân) are recited unto them, they (i.e. the Verses) increase their Faith; and they put their trust in their Lord (Alone) [Surah Anfaal 8:2]

This is one of many verses mentioning the increase in eemaan. Others are 74:31, 48:4, 3:173, 9:124

From the Sunnah:

The hadeeth: "The most perfect of people with respect to his eemaan, is the best of them in his manners." (Bazzaar, all of its narrators are reliable and al-Haithamee mentioned it in Majmoo az-Zawaa’id)

And the hadeeth: "Whoever loves for Allaah (alone), hates for Allaah (alone), gives for Allaah (alone) and withholds for Allaah (alone), then he has perfected eemaan." (Abu Daawood - saheeh).

These two hadeeth show that eemaan can be complete or incomplete and therefore it can go up (in order to be complete) and it can go down (so that it is incomplete). Notice how the first hadeeth contains actions of the tongue and actions of the limbs (speaking and behving well with people) and how the second hadeeth contains both actions of the heart (loving and hating) and actions of the limbs (giving and withholding).

The hadeeth: "Eemaan consists of seventy-odd branches, the highest of which is to testify ‘None has the right to be worshipped except Allaah’ and the lowest of which is to remove something harmful from the floor, and modesty is a part of eemaan." (Bukhaaree, Muslim Abu Daawood, Ahmad, Ibn Maajah, and others.)

Did you notice how this hadeeth contains "speech" and "action"? It mentions the testification ‘None has the right to be worshipped except Allaah’ which is the speech of the heart and the speech of the tongue (To testify to something means to have firm knowledge of it - which is in the heart - and to believe that this knowledge is true and correct, and then also to speak with it so that you are ‘testifying’ to it). And it mentions some actions, one which is to do with the limbs (removing something harmful from the floor) and one which is to do with the heart (modesty).

From the Salaf:

Al-Laalikaa'ee (d. 418H) reports in his Sharh Usool I'tiqaad Ahl is-Sunnah (5/958) that Abdur-Razzaaq (as-San'aanee) said: "I met sixty two Shaikhs, amongst them were: Ma'mar, al-Awzaa'ee, ath-Thawree, al-Waleed bin Muhammad al-Qurashee, Yazeed ibn as-Saa'ib, Hammaad bin Salamah, Hammaad bin Zaid, Sufyaan bin Uyainah, Shu'ayb bin Harb, Wakee' bin al-Jarraah, Maalik bin Anas, Ibn Abee Laylaa, Isma'eel bin Ayyaash, al-Waleed ibn Muslim and those I have not named, all of them saying: "Faith consists of speech and action, it increases and decreases."