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
Question

Is speaking about things one does not know properly a sin?
Is speaking about things one does not remember correctly a sin?
Is speaking about things one does not know at all a sin?
Praise be to Allah.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)
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.