Mishkat al-Masabih

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 corruption, then not disclosing certain matters is also permissible, but at an important occasion or when a necessary statement 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 knowledge (which was related to political matters), which he did not disclose to the people. See: [صحيح بخاري : 120]
➌ The chain of the narration 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.

Commentary:
(1)
To conceal means that he knew the correct ruling, yet without any valid excuse, he did not disclose it.

(2)
"Lijam" in the Arabic language refers to that part of the bridle which is placed in the mouth of a horse or similar animal and is made of iron.
The part of the bridle that is held in the rider’s hand is called "zamaam."

(3)
From this, the severe punishment for concealing knowledge is established.