The Muslims did not disagree that Allah, may He be exalted, has preserved His Book from additions and subtractions, in fulfilment of His words (interpretation of the meaning):

“Verily We: It is We Who have sent down the Dhikr (i.e. the Quran) and surely, We will guard it (from corruption)”

[al-Hijr 15:9]. 

The Muslims are unanimously agreed that anyone who disagrees with this and claims that there is something in the Book of Allah that is not part of it, or that something that Allah revealed has been taken away from it, is a disbeliever. 

Al-Qaadi ‘Iyaad (may Allah have mercy on him) said: 

The Muslims are unanimously agreed that the Qur’aan which is recited in all parts of the world and is written in the Mushaf that the Muslims have, that is contained between the two covers, from the first soorah “All the praises and thanks be to Allah, the Lord of the Alameen (mankind, jinns and all that exists)” to the last soorah, “Say: "I seek refuge with (Allah) the Lord of mankind”, is the Book of Allah and His Revelation that He sent down to His Prophet Muhammad (blessings and peace of Allah be upon him), and that everything in it is true, and that anyone who deliberately takes away a single letter from it or changes one letter for another or adds a letter to it, that is not included in the Mushaf on which there is consensus, and there is consensus that it is not part of the Qur’aan – the one who does any of these things deliberately is a disbeliever. End quote. 

ash-Shifa bi Ta‘reef Huqooq al-Mustafa, 2/304, 305 

With regard to the difference of opinion among the scholars as to whether the Basmalah is counted as a verse of the Qur’aan or not, this does not come under this heading, because the scholars of Qur’aan recitation did not differ with regard to reciting it at the beginning of the soorahs. And the Sahaabah (may Allah be pleased with them) were unanimously agreed that it must be written at the beginning of all soorahs apart from Soorat at-Tawbah, and that is in the Mushaf that was written by ‘Uthmaan ibn ‘Affaan (may Allah be pleased with him) and sent to all regions. 

Ash-Shawkaani (may Allah have mercy on him) said, when discussing the Basmalah: 

It should be noted that the ummah is unanimously agreed that neither the one who affirms it nor the one who denies it is to be described as a disbeliever, because of the difference of scholarly opinion concerning it. This is different from the case of a person who denies a letter on which there is consensus or who affirms something that no one said; he is to be regarded as a disbeliever according to scholarly consensus. 

There is no difference of opinion concerning the fact that it is a verse in Soorat an-Naml, or that it is a line to be written at the beginning of the soorahs in the Mushaf, apart from the beginning of Soorat at-Tawbah. 

Nayl al-Awtar, 2/215 

Some of the scholars regard the difference of opinion as to whether the Basmalah is a verse of the Qur’aan as being like the difference of opinion among the leading scholars of recitation about some words and letters that may be proven in some recitations and not in others. 

Shaykh Muhammad al-Ameen ash-Shanqeeti (may Allah have mercy on him) said: 

The scholars differed concerning the Basmalah: is it a verse at the beginning of each soorah, or only at the beginning of al-Faatihah, or is it not a verse at all? As for the words of Allah in Soorat an-Naml, “Verily! It is from Sulaiman (Solomon), and verily! It (reads): In the Name of Allah, the Most Beneficent, the Most Merciful” [an-Naml 27:30], this is a verse of the Qur’aan according to scholarly consensus. 

With regard to Soorah Baraa’ah (at-Tawbah), the Basmalah is not a verse thereof, according to scholarly consensus. But there is a difference of scholarly opinion concerning other cases. Some of these scholars of Usool stated that the Basmalah is not part of the Qur’aan, and some said that it is part of it in al-Faatihah only. And it was said that it is a verse at the beginning of every soorah. This is the view of ash-Shaafa‘i (may Allah have mercy on him). 

One of the best things that have been said concerning this matter, to reconcile between all the opinions, is that in some recitations, such as the recitation of Ibn Katheer, the Basmalah is a verse of the Qur’aan, and in some other recitations it is not a verse, and there is nothing strange about that. 

In the verse in Soorat al-Hadeed, “Fa inna Allaha huwa al-Ghaniy ul-Hameed (then Allah is Rich (Free of all wants), Worthy of all praise)” [al-Hadeed 57:24],  the word “huwa” is part of the Qur’aan according to the recitations of Ibn Katheer, Abu ‘Amr, ‘Aasim, Hamzah, and al-Kasaa’i, and it is not part of the Qur’aan according to the recitations of Naafi‘ and Ibn ‘Aamir, because they recited it “Fa inna Allaha al-Ghaniy al-Hameed”. And in some Mushafs the word “huwa” is present and in others it is not present. 

In the passage “so wherever you turn yourselves or your faces there is the Face of Allah (and He is High above, over His Throne). Surely! Allah is All-Sufficient for His creatures needs, All-Knowing. And they (Jews, Christians and pagans) say: Allah has begotten a son (children or offspring)” [al-Baqarah 2:115-116], the waw (in the phrase wa qaalu attakhadha Allahu waladan/and they say: Allah has begotten a son) is present in the recitation of the seven, except for Ibn ‘Aamir; according to the recitation of Ibn ‘Aamir it is not part of the Qur’aan, because he recited it as “Qaalu attakhadha Allahu waladan (They say Allah has begotten a son)”, without the waw. So it is omitted in the Mushaf of the people of ash-Shaam (greater Syria), and there are other exaples. 

Thus it is known that it does not matter whether the Basmalah is regarded as a verse in some recitations and not others. The scholars are agreed on this point.

End quote from al-Madhkarah fi Usool al-Fiqh, p. 66, 67. 

And Allah knows best.