The seven modes of recitation are mutawaatir and it is not permissible to cast aspersions on them

Praise
be to Allah

Firstly:

The majority of scholars of recitation and usool are of the
view that the seven modes of recitation are mutawaatir going back to the
Prophet (blessings and peace of Allah be upon him). Some of them differed
concerning that, such as Abu Shaamah – according to one of his views – and
at-Tawfi and ash-Shawkaani. However the correct view concerning that is the
view of the majority; that is the right view which must be adopted.

Shihaab ad-Deen ad-Dimyaati (may Allah have mercy on him)
said:

Taaj al-A’immah as-Subki – who is Taaj ad-Deen ‘Abd al-Wahhaab
as-Subki – said in his fatwas:

The seven modes of recitation to which ash-Shaatibi referred,
to the exclusion of others, and the three others – namely the recitation of
Abu Ja‘far, the recitation of Ya‘qoob and the recitation of Khalaf – are
mutawaatir and well-known, and it is an established fact in the religion
that they were revealed to the Messenger of Allah. No one would stubbornly
disagree with that except one who is ignorant. They are not only regarded as
mutawaatir by those who know these ten modes of recitation; rather they are
regarded as mutawaatir by every Muslim who says “I bear witness that there
is no god but Allah and I bear witness that Muhammad is the Messenger of
Allah,” even if he is rough and uneducated and has not memorised even one
letter of the Qur’an.

He said: We could elaborate on the proof and evidence, but we
do not have room to discuss this matter at length here. However, what is
expected of every Muslim, and what he should do is to accept before Allah,
may He be exalted, and be certain that what we have mentioned is mutawaatir
and is known for certain, and there is no room for doubt or uncertainty
about it.

Conclusion:

The seven modes of recitation are mutawaatir according to
consensus, as are the three others: the recitations of Abu Ja‘far, Ya‘qoob
and Khalaf, according to the more correct view. In fact the correct,
favoured view, which is what we learned from most of our shaykhs, is that
the recitations of the other four – Ibn Muhaysin, al-Yazeedi, al-Hasan and
al-A‘mash, are odd (shaadhdh), according to scholarly consensus.

An-Nawawi (may Allah have mercy on him) said:

Our companions and others said:

It is permissible to recite, in prayer and otherwise,
according to any of the seven modes of recitation, but it is not permissible
to recite, in prayer or otherwise, according to an odd mode of recitation,
because that is not Qur’an. Qur’an can only be proven by means of mutawaatir
reports, and each of the seven modes of recitation is mutawaatir. This is
the correct view which one must accept, and whoever says otherwise is wrong
or ignorant.

End quote from al-Majmoo‘ (3/392)

Ibn an-Najjaar al-Fatoohi (may Allah have mercy on him) said:

The seven modes of recitation are mutawaatir according to the
four imams and other leading Sunni scholars. This was narrated by as-Sarakhsi,
who was one of the companions of ash-Shaafa‘i, in Kitaab as-Sawm in
al-Ghaayah. He said: The Mu‘tazilah said: (The modes of recitation
are transmitted via) aahaad reports.

[An aahaad hadith is one of which the narrators at each stage
of the isnaad are too few for it to be regarded as mutawaatir.]

End quote from Sharh al-Kawkab al-Muneer (2/127)

Az-Zarqaani (may Allah have mercy on him) said:

The fact that is supported by evidence is that the ten modes
of recitation are all mutawaatir. This is the view of the scholars of usool
and of recitation, such as Ibn as-Subki, Ibn al-Jazari and an-Nuwayri.

End quote from Manaahil al-‘Irfaan (1/441)

Secondly:

There is a specious argument that may enter some people’s
minds, which says that the isnaads of the recitations mentioned by the imams
in their books are well known and limited, but they are ahaad isnaads, so
how can it be said that their recitations are mutawaatir?

The response to that is: this is an old specious argument,
which was answered by more than one of the scholars of recitation and usool.
The leading scholars who specialised in this field, on whose teachings these
modes of recitation are based, and to whom the isnaads go back, are not the
only ones who were proficient in these modes of recitation or the only ones
who specialised in them. Rather these modes of recitation were mutawaatir
during their time, and remained so after their time, as there were other
scholars like them who also specialised in that field and had other students
who learned from them this branch of knowledge, which has to do with the
modes of recitation and the rules and regulations governing them.

Imam Sharaf ad-Deen ad-Dimyaati (may Allah have mercy on him)
said:

The fact that the isnaads that have to do with the modes of
recitation are limited to one group does not mean that the modes of
recitation did not have other isnaads. Rather the reports that have to do
with the modes of recitation are attributed to them because they are the
ones who became specialised in that matter and recorded the names of their
shaykhs from whom they learned it, and each of them [the narrators in the
isnaads] in his generation had many others who specialised in that field,
which would raise the reports to the level of being mutawaatir.

This is the view of the scholars, and the dissenting view of
Ibn al-Haajib, who differed with regard to some of what we have mentioned,
was refuted by one who was very well versed in this field, namely Ibn al-Jazari,
who refuted it at length in his book al-Munjid, which is worth
reading.

End quote from Ithaaf Fudala’ al-Bashar fi’l-Qiraa’aat al-Arba‘at
‘Ashar (p. 9)

Ibn an-Najjaar al-Fatoohi (may Allah have mercy on him) said:

Some said that the reports concerning the modes of recitation
were aahaad reports, such as at-Tawfi in his Sharh. He said: The fact
is that they (the modes of recitation) are widely known as being attributed
to them, but they were not widely narrated from them, because the isnaads of
the seven imams of recitation for the seven modes of recitation going back
to the Prophet (blessings and peace of Allah be upon him) are to be found in
the books of recitation, which were transmitted from one scholar to another
at a time, so they do not fulfil the conditions of being mutawaatir.

The response to that is as follows: the fact that the isnaads
are limited to a particular group does not mean that the modes of recitation
were not transmitted via other narrators. The inhabitants of every city used
to learn recitation according to the recitation of their Imam, who was one
of the Sahaabah or otherwise, so a large number of people would learn from
another large number like them, and so on. Therefore the transmission of the
recitations was mutawaatir, but the imams who became specialised in the
modes of recitation recorded the names of their shaykhs from whom they
learned their mode of recitation, and that is why they made their isnaads
available (whilst others did not). This is like the reports about the
Farewell Pilgrimage which are known to be aahaad, but the reports of the
Farewell Pilgrimage continued to be transmitted in the manner of mutawaatir
reports, from one large number of people to another, throughout the ages. So
one should pay attention to that, and not be deceived by the arguments of
those who say that the isnaads of the scholars of recitation are aahaad.

End quote from Sharh al-Kawkab al-Muneer (2/127-128)

Thirdly:

With regard to the questioner’s asking: how can we say that
(the modes of recitation) are mutawaatir when the term mutawaatir did not
appear before Ibn Mujaahid who was the seventh of seven?

The response to that is as follows:

What is meant by something being mutawaatir is learning
something on the basis of certainty when there is no doubt at all, as
opposed to learning something on the basis of probability. The level of
certainty concerning the soundness of the seven modes of recitation is
established, and it was transmitted by way of mutawaatir reports, which
effectively gives certainty. And that is the point, regardless of whether
this terminology was known at that time or not. This is simply the matter of
terminology that has to do with setting out guidelines in various fields of
knowledge and developing them. The Arabs (before Islam) used to observe the
rules of grammar in their language, giving words case endings according to
whether words were nominative, accusative or genitive, and so on, but if the
people of the Jaahiliyyah were to come back to life now, none of them would
know the terminology of Arabic grammar; in fact none of them knew anything
about grammar as people speak of it now.

Similarly, with regard to knowledge of hadith, usool al-fiqh
and other branches of Islamic knowledge, the fact that the specific
terminology of any particular field appeared later on does not mean that the
ideas and concepts that the terminology refers to were not known previously
to specialists in that field.

As for the notion that the concept was unknown among people
who were not specialists, or that it was not widely used by others in
general, that does not undermine the fact that the concept was there, and it
does not undermine the soundness of a particular academic issue.

Imam Shams ad-Deen adh-Dhahabi (may Allah have mercy on him)
said:

The fact that something is mutawaatir does not necessarily
mean that the entire ummah is aware of it. For the scholars of recitation,
there are matters that are mutawaatir among them, but not among others. For
the fuqaha’ there are issues that are mutawaatir among them and narrated
from their imams via mutawaatir reports, of which the scholars of recitation
are unaware. Among the hadith scholars, there are mutawaatir hadiths that
the fuqaha’ have never heard, or at least for the fuqaha’ they were not
mutawaatir; rather they were at the level of probability only. For the
grammarians there are definitive issues, and the same is true of the
linguists. End quote.

Siyar A‘laam an-Nubala’
(10/171)

See also the answer to question no. 5142

And Allah knows best.