Imaan an-Nawawi said in "Kitaab ur-Riddah" (The Book of Apostasy), "It is the cutting off of Islaam. This occurs sometimes by a statement which constitutes kufr, and sometimes by an act. And the actions which necessitate kufr (that expel from the religion) are those which are performed deliberately, and mocking the religion is clear [in this regard], such as prostrating to an idol or to the sun, or throwing the Qur-an into filth, and the magic which involves worshipping the sun and other such acts."
Ibn al-Qayyim said, "And likewise, the branches of kufr (amongst them are) those which are statements and those which are actions. And just like a person can fall into disbelief (that expels from the religion) by wilfully saying a word constituting disbelief, and which is a branch of disbelief, then likewise, he can fall into disbelief (that expels from the religion) by doing an act of kufr, from amongst its branches that are actions, such as prostrating to an idol, belittling the mus-haf. So this is a principle"
Shaikh 'Abdur-Rahman bin Hasan said, "And likewise the Fuqahaa have mentioned concerning the ruling upon the apostate that a person can become a disbeliever by a statement (he utters) or an action he performs, even if he testifies that there is nothing which has the right to be worshipped except Allaah and Muhammad is His Messenger, prays, fasts and gives charity. He becomes an apostate by such statements and actions and whatever he uttered or did nullifies his actions, especially if he dies upon that. In that case the nullification of his actions is a matter of consensus."
Shaikh 'Abdur-Rahman bin Nasir as-Sa'dee said, "Chapter: The Ruling upon the Apostate. "The apostate, murtad, is the one who exits from Islaam and enters into disbelief on account of an action, statement, a belief (i'tiqad) or doubt (shakk)."
Shaikh Ibn Uthaimeen said, "And whoever Allaah and His Messenger declares to be a disbeliever, then he is a disbeliever, regardless of whether that is by way of an action (he committed), or a belief (that he held) or a statement that he uttered. And whoever Allaah and His Messenger have not declared to be a disbeliever, and he ascribes himself to Islaam, then he is a believer and it is not lawful for us to declare him a disbeliever."
Imaam al-Albani said, "And amongst the actions are those on account of which a person actually disbelieves with the kufr of belief (i.e. apostatises). This is because such actions show his disbelief with absolute certainty and decisiveness in the sense that when a person commits them, it is as if he is actually expressing his disbelief with his tongue, such as the one who kicks the Qur'an while knowing it is the Qur'an and intending to kick it, deliberately (qasd)." 
Shaikh Khalid al-'Anbari said, "The Extremist Murji'ah opposed Ahl us-Sunnah when they made Imaan to be mere tasdeeq (assent of the heart)... and that it is possible for perfect Imaan to be in the heart while a person utters kufr and reviles Allaah or the Messenger, deliberately without compulsion, and that those outward statements on account of which a person becomes a disbeliever, then the reason for that is that they necessitate the absence of tasdeeq (assent of the heart)... Hence, they made kufr restricted to the rejection (takdheeb) of the heart, and they thought that whoever was declared a disbeliever by the Legislator was declared so because the tasdeeq (assent) of the heart had disappeard from him... and the Salaf declared those who spoke with this saying to be disbelievers, for Iblees the Accursed is a disbeliever by the textual evidence of the Noble Qur'an but was not a rejector (mukaddhib), rather he was a wilful resistor (mu'aanid) and arrogant (mustakbir)" 
Shaikh Abdul-Azeez Bin Baz said, "Sacrificing for other than Allaah and prostrating to other than Allaah is the kufr of action that expels from the religion. Similarly, if a person was to pray to other than Allaah or prostrate to other than Him, free is he of all imperfections, then he would disbelieve with the major kufr of action - and refuge is from Allaah. Likewise, if he was to abuse the religion, or abuse the Messenger (sallallaahu alaihi wasallam), or mock Allaah or the Messenger, then that is the major kufr of action in the view of all of Ahl us-Sunnah wal-Jamaa'ah." 
As stated above, that with which we worship our Lord is that amongst the statements and actions are those which are kufr (that expels from the religion). Actions and statements of kufr are in and of themselves kufr. Either the minor kufr (or kufr of ni'mah) such as the sins that do not negate a person's Imaan (i.e. Islaam) totally like fornication, stealing, lying, wailing over the dead by way of example or the major kufr that expels from the religion such as reviling Allaah or the Messenger, kicking the Qur'an, prostrating to an idol by way of example only. This, in opposition to the Muri'ah who restrict kufr (and Imaan) to the heart only.
 Rawdat ut-Taalibeen (7/284-283)
 Kitaab us-Salaat (pp.53-54)
 Ad-Durar as-Sunniyah (p.214)
 Manhaj us-Saalikeen (p.112)
 Cassette: Questions and Answers from Qatar (30th April, 2000)
 Fitnah of Takfir (p.72 1st Edition). And this is a refutation of the one who claims that Imaam al-Albani performs takfir only by istihlaal qalbi (making an act of kufr lawful with one's heart), which is a great slander indeed.
 Al-Hukmu Bi-Ghayri Maa Anzallallaahu wa Usul ut-Takfir (p.18).
 Al-Furqan (no. 94).