The basic principle concerning the Muslim’s speech is that it should be truthful and honest; he should not speak on the basis of speculation, or about that of which he has no knowledge

Firstly:

The Muslim should strive to speak the truth and be honest,
and his words should be useful, whether they are about worldly matters or
matters of the hereafter.

He should not speak too much about that which is of no
benefit and serves no purpose, and he should avoid speaking on the basis of
speculation or speaking of that of which he has no knowledge, because this
is what it means to be truthful and honest. Allah, may He be glorified and
exalted, had instructed His believing slaves to be with those who are true
(in word and deed), as He says (interpretation of the meaning):
“O you
who believe! Fear Allah, and be with those who are true (in words and deeds)”
[at-Tawbah 9:119].

Ibn Katheer (may Allah have mercy on him) said:

Be true and adhere to truthfulness and be with those who are
true, and you will be saved from doom and be granted relief from hardship.
End quote.

Tafseer Ibn Katheer
(4/230)

As-Sa‘di (may Allah have mercy on him) said:

“and be with those who are true”
that is, those who are true in word, deed
and conduct, those whose words are true and whose deeds and conduct cannot
be but based on truth, devoid of laziness and apathy, free of bad
intentions, and based on sincerity and good intentions. For truthfulness
leads to righteousness, and righteousness leads to Paradise. End quote.

Tafseer as-Sa‘di
(p. 355)

Abu Dawood (4989) and at-Tirmidhi (1971) narrated that Ibn
Mas‘ood said: The Messenger of Allah (blessings and peace of Allah be upon
him) said: “Beware of lying, for lying leads to wickedness and wickedness
leads to Hell. A man may lie and strive hard in lying until he is recorded
with Allah as a liar. You should be truthful for truthfulness leads to
righteousness and righteousness leads to Paradise. A man may speak the truth
and strive hard in speaking the truth until he is recorded with Allah as a
speaker of truth.”.

Classed as saheeh by al-Albaani in
Saheeh Abi Dawood

Allah and His Messenger (blessings and peace of Allah be upon
him) have forbidden us to engage in speculation or suspicion. Allah, may He
be exalted, says (interpretation of the meaning):

“O you who
believe! Avoid much suspicion, indeed some suspicions are sins”

[al-Hujuraat 49:12].

Al-Bukhaari (5143) and Muslim (2563) narrated from Abu
Hurayrah that the Prophet (blessings and peace of Allah be upon him) said:
“Beware of suspicion, for suspicion is the falsest of speech.”

And Allah, may He be exalted, says (interpretation of the
meaning):

“And follow
not (O man i.e., say not, or do not or witness not, etc.) that of which you
have no knowledge”

[al-Isra’ 17:36].

Ibn Katheer (may Allah have mercy on him) said:

Qataadah said: Do not say, “I saw” when you did not see, or
say “I heard” when you did not hear, or “I know” when you do not know, for
verily Allah will question you about all of that.

What we may conclude from their comments is that Allah, may
He be exalted, forbids speaking without knowledge; in fact He forbids
speculation or suspicion, which is based on imagination. End quote.

Tafseer Ibn Katheer
(5/75)

Al-Qutaybi said: Do not speak on the basis of speculation and
suspicion.

Tafseer al-Baghawi
(5/92)

As-Sa‘di (may Allah have mercy on him) said:

That is, do not speak of that of which you have no knowledge;
rather make sure that everything that you say or do  is based on proper
knowledge, and do not think that you will be let off for that.  End quote.

Tafseer as-Sa‘di
(p. 457)

The prohibition on such talk is more emphatic when it comes
to matters of Islamic teachings and rulings on halaal and haraam. It is not
permissible for anyone to speak about the religion of Allah without
knowledge, or on the basis of speculation and conjecture. Imam Ahmad (6702)
narrated from ‘Abdullah ibn ‘Amr ibn al-‘Aas (may Allah be pleased with him)
from the Prophet (blessings and peace of Allah be upon him) that he said:
“The Qur’an was not revealed to contradict itself; rather parts of it
confirmed other parts. Whatever you understand of it, act upon it, and
whatever you do not understand of it, refer it to one who has knowledge of
it.”

It was classed as saheeh by the commentators on al-Musnad.

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said:

Whatever a person knows, he is obliged to follow it and act
upon it, because in his case that is like a leader. Whatever he is ignorant
of, such as that which he is confused about and does not know its meaning,
he should leave it to one who has knowledge of it. End quote.

Bayaan Talbees al-Jahamiyyah
(8/377)

See also the answer to question no.
126198

Based on that:

The Muslim should not speak about anything of which he does
not have sufficient knowledge, or anything of which he does not remember the
details correctly, or anything of which he has no knowledge. Rather he
should speak – if he does speak – on the basis of knowledge, otherwise
remaining silent is safer. Allah does not require him to speak on the basis
of speculation or suspicion, which is the falsest of speech, or to speak of
that of which he does not have sufficient knowledge.

The Muslim may sometimes be required to speak on the basis of
speculation and what he thinks is the case or of that which he does not
remember well. In that case he should explain that to his listener, and tell
him that he is only speaking on the basis of speculation or what he thinks
is the case, and not on the basis of certain knowledge.

Conclusion:

The Muslim should not speak of anything except that which he
knows for certain, and he should avoid speaking on the basis of speculation,
conjecture and possibilities, except within the limits dictated by
interests, when one may sometimes speak in that manner.

And Allah knows best.