Lying in buying and selling.
It was narrated from Abu Dharr (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: "There are three to whom Allaah will not speak on the Day of Resurrection and He will not look at them or praise them, and they will have a painful torment." The Messenger of Allaah (peace and blessings of Allaah be upon him) repeated this three times. Abu Dharr said: "May they be doomed and lost, who are they, O Messenger of Allaah?" He said, "The one who lets his garment hang below his ankles, the one who reminds others of his favours, and the one who disposes of his goods by swearing a false oath."
(Narrated by Muslim, 106)

It was narrated that Hakeem ibn Hizaam (may Allaah be pleased with him) said: "The Messenger of Allah (peace and blessings of Allah be upon him) said: 'The two parties involved in a transaction have the option [of cancelling it] until they part. If they are honest and truthful, their transaction will be blessed for them and if they conceal something and tell lies, the blessing of their transaction will be wiped out.'"
(Narrated by al-Bukhaari, 1973; Muslim, 532)

Shaykh al-Islam Ibn Taymiyah said:
"Allaah has enjoined truthfulness and honesty, and He has forbidden lies and concealment with regard to matters which should be known and revealed to people, as the Prophet (peace and blessings of Allah be upon him) said in a hadeeth whose authenticity is agreed upon: 'The two parties involved in a transaction have the option [of cancelling it] until they part. If they are honest and truthful, their transaction will be blessed for them and if they conceal something and tell lies, the blessing of their transaction will be wiped out.' And Allaah says (interpretation of the meaning):

'O you who believe! Stand out firmly for Allaah as just witnesses; and let not the enmity and hatred of others make you avoid justice. Be just: that is nearer to piety'
[al-Maa'idah 5:8]."
(Minhaaj al-Sunnah, 1/16)

3. The prohibition on lying about visions and dreams:
This refers to when some of them claim to have seen such and such in a dream, but they are not telling the truth, then in the morning he starts to tell the people something that he did not see.
It was narrated from Ibn 'Abbaas that the Prophet (peace and blessings of Allaah be upon him) said: "Whoever tells of a dream that he did not see will be commanded [on the Day of Resurrection] to tie two grains of barley together, but he will never be able to do it. Whoever eavesdrops on people's conversation when they dislike that - or they try to get away from him - will have molten copper poured in his ears in the Day of Resurrection. And whoever makes an image will be punished and will be told to breathe life into it, and he will not be able to do so."
(Narrated by al-Bukhaari, 6635)

Al-Manaawi said:
" 'to tie two grains of barley together, but he will never be able to do it' - because tying one to the other is not possible ordinarily. He will be punished until he does that, and he will never be able to do it. It is as if he is saying that he will be enjoined to do something that he can never do, and is being punished for it. This is a metaphor for continuous punishment... The reason why barley (sha'eer) is mentioned specifically is because dreams are connected to feelings (shu'oor); the words sha'eer (barley) and shu'oor (feelings) come from the same root in Arabic.
The punishment for that is so severe - even though lies when awake may cause more damage, because they may involve testimony that may lead to execution or a hadd punishment - because lying about a dream is a lie about Allaah, may He be exalted, for dreams are a part of Prophethood, so they come from Him, and lying about the Creator is worse than lying about created beings.
(Fayd al-Qadeer, 6/99)

4. The prohibition of speaking about everything that one hears:
It was narrated that Hafs ibn 'Aasim said: "The Messenger of Allaah (peace and blessings of Allaah be upon him) said: 'It is sufficient lying for a man to speak of everything that he hears.'"
(Narrated by Muslim, 5)

With regard to the meanings of the hadeeth and reports on this topic, they discourage speaking about everything that one hears. For usually a person will hear both true things and lies, so if he were to speak of everything that he hears, he would be lying, by telling of something that did not happen. We have stated above the view of the people of truth: that lying means telling of something in a manner other than the way it happened. It is not conditional upon it being done deliberately, but its being done deliberately is a condition of it being a sin. And Allaah knows best."
(Sharh Muslim, 1/75)

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A teacher by profession