Tirmidhi

Explanation:
1:
From this hadith, it is understood that not informing someone who asks about religious knowledge of the correct matter of the religion, or denying them the means to acquire religious knowledge (such as books, etc.), is a cause of severe warning and sin.
Similarly, concealing the correct rulings of the religion despite having knowledge of them, and not clarifying them before the people, is also sinful.

Benefits and Issues:
Benefit: According to Fudayl ibn ‘Iyad, this pertains to the obligatory duties (fara’id).
Learning them is obligatory upon every common Muslim.
Therefore, it is obligatory upon the scholar to teach them.
As for those matters which are not obligatory,
teaching them is also not obligatory.
And Allah knows best.

If an issue is not known, to fabricate and state it based on one’s own opinion is also a grave sin. However, if after searching it is not found in the Qur’an or Hadith, then exercising ijtihad (independent juristic reasoning) is permissible.

Commentary:

➊
Imam Khattabi rahimahullah said:
The knowledge intended here is that which is absolutely necessary for the questioner to know, for example:
the method of performing prayer (salah), etc.
This warning does not apply to supererogatory (nafl) sciences, such as:
the knowledge of grammar (nahw), morphology (sarf), or logic and philosophy.

➋
Sometimes an issue is above the intellectual level of the questioner, making it difficult for him to understand, for example:
an ordinary person who can only comprehend that a hadith is either authentic (sahih) or weak (da'if).
If he asks about the reasons for weakness or about the statements of the scholars of criticism and praise (jarh wa ta'dil) regarding a narrator, then it is permissible to tactfully avoid answering, just as when the Prophet sallallahu alayhi wa sallam was asked about the waxing and waning of the moon, Allah Most High, instead of explaining the reason, mentioned its wisdom and benefit.
It is stated:
﴿يَسْـَٔلُونَكَ عَنِ ٱلْأَهِلَّةِ ۖ قُلْ هِىَ مَوَ‌ٰقِيتُ لِلنَّاسِ وَٱلْحَجِّ﴾ (al-Baqarah: 189)
"People ask you about the new moons. Say: They are measurements of time for the people and for Hajj."
Similarly, when asked about the reality of the soul (ruh), it was said:
﴿قُلِ ٱلرُّ‌وحُ مِنْ أَمْرِ‌ رَ‌بِّى﴾ (Bani Isra'il: 85)
"Say: The soul is from the command of my Lord."
That is, by Allah's command, a thing has been created whose reality you cannot comprehend.

➌
If it is feared that a person will misuse knowledge, then it is also permissible to refrain from answering him.
Hajjaj bin Yusuf asked Anas bin Malik radi Allahu anhu: What is the harshest punishment the Messenger of Allah sallallahu alayhi wa sallam ever gave? He narrated the incident of the 'Uraniyyin, in which some people from the tribe of 'Uraina came to Madinah, but the climate did not suit them and they became ill.
The Prophet sallallahu alayhi wa sallam prescribed for them the milk and urine of camels as treatment.
He sent them to the camels of charity so that they could treat themselves.
When they recovered, they stole the camels of the Messenger of Allah sallallahu alayhi wa sallam and brutally killed his shepherd.
The Messenger of Allah sallallahu alayhi wa sallam sent a group of Companions radi Allahu anhum to capture them, and they were brought back.
The Prophet sallallahu alayhi wa sallam had their hands and feet cut off, had hot iron rods passed over their eyes, and left them in the sun thirsty until they died writhing in agony.
They had killed the Prophet's shepherd in the same manner, so they were punished according to their deed. (Sahih al-Bukhari, Book of Ablution, Chapter: The urine of camels and other animals...etc., Hadith: 233)

Anas bin Malik radi Allahu anhu used to say:
I wish I had not narrated this hadith, because Hajjaj bin Yusuf used this hadith as evidence to inflict severe tortures on people. (See Tafsir Ibn Kathir, Surah al-Ma'idah: 33)

➍
When a question is asked for the purpose of testing someone, in order to accurately assess his scholarly ability, then the one being questioned should be given the opportunity to answer according to his own knowledge, and it is not correct for another person to assist him, because this defeats the very purpose of the test.
Sometimes, because of this, a deserving person is deprived of his right, and an unqualified person attains a position he does not deserve.
Succeeding in an exam by using illegitimate means falls under this warning. (al-mutashabbi‘ bima lam yu‘ta, kalabis thawbay zur) (Sahih Muslim, Book of Dress, Chapter: The prohibition of forgery in dress and other matters, and pretending to have what one does not possess, Hadith: 2129)
"Whoever does not possess a thing and pretends to have it, it is as if he has put on two garments of falsehood."

Authentication of the Hadith:
This hadith is hasan (good).
It has been declared hasan by al-Tirmidhi, and authentic by Ibn Hibban [الاحسان : 95], [حاكم 1؍1۔ 1 ح344], and al-Dhahabi.
◄ In this narration of Ata bin Abi Rabah, there is discussion regarding his hearing from Sayyiduna Abu Hurairah radi Allahu anhu, although in al-Mustadrak al-Hakim, in the narration of Qasim bin Muhammad bin Hammad, explicit mention of his hearing is present.
◄ Three hadith scholars have criticized Qasim bin Muhammad al-Dallal, and three have declared him trustworthy. In Sahih Ibn Hibban [الاحسان : 96] and al-Mustadrak lil-Hakim [1؍1۔ 2 ح346 وصححه] and others, there is a hasan li-dhatihi (intrinsically good) corroborating narration: «من كتم علما ألجمه الله يوم القيامة بلجام من نار.», due to which this is hasan. «والحمد لله»

Jurisprudence of the Hadith:
➊ Concealing the truth is haram (forbidden).
➋ It is established from other evidences that if there is a legitimate Islamic excuse, then it is also permissible not to disclose certain matters of knowledge before the general public. For example:
① The Prophet sallallahu alayhi wa sallam told Sayyiduna Mu'adh bin Jabal radi Allahu anhu a certain matter, then forbade him from telling it to the people, lest they rely solely upon it. See: [صحيح بخاري 128 - 129] and [صحيح مسلم 32]
② Sayyiduna Abu Bakr al-Siddiq radi Allahu anhu made a mistake in the interpretation of a dream, regarding which the Messenger of Allah sallallahu alayhi wa sallam said:
«أصبت بعضا وأخطأت بعضا.»
"Some of your statements are correct and in some you have erred." Abu Bakr al-Siddiq radi Allahu anhu said: By Allah! You must tell me in what I erred? So the Prophet sallallahu alayhi wa sallam said: "Do not swear an oath." [صحيح بخاري : 7046،صحيح مسلم : 2269]
That is, he did not inform him of his mistake. Therefore, if there is an excuse or fear of harm, then not disclosing certain matters is also permissible, but at an important occasion or when a necessary clarification is required, concealing knowledge is not permissible; rather, due to being concealment of the truth, it is haram.
③ The Messenger of Allah sallallahu alayhi wa sallam told Sayyiduna Abu Hurairah radi Allahu anhu a certain knowledge (which was related to political matters), which he did not disclose to the people. See: [صحيح بخاري : 120]
➌ The chain of narration of the report in Sunan Ibn Majah [264] is weak due to Yusuf bin Ibrahim, but due to the aforementioned hasan corroborating narration in Sahih Ibn Hibban [96], this narration is also hasan.