The Ruling on One Who Withholds Knowledge

Question:
He also asks and says: A person was asked about something he knows, so he swore by the Great Allah that he does not know about that matter, and he is now deeply regretful of what he did. What is the expiation for that? And what is the thing that is obligatory for him to do? May Allah reward you with good.

Answer:
He must repent to Allah from... he must repent from withholding the knowledge. The Prophet (peace and blessings be upon him) said: Unless that matter is one where one fears that informing about it will lead to fitnah (trial, discord or temptation) and great evil, in which case he is excused. If withholding the knowledge is such that the benefit in withholding it is greater than the benefit in revealing it, then there is no harm. Just as the Prophet (peace and blessings be upon him) said to Mu'adh when he informed him of the right of Allah over the servants and the right of the servants over Allah. Mu'adh said: Should I not inform the people? He said: Then Mu'adh informed them of it, and the Prophet (peace and blessings be upon him) informed of it, and he conveyed it to the Ummah (the Muslim community) to convey the proof and to establish the proof.
So the point is: If your informing about what you were asked about leads to evil that you see as great and dangerous, then you are excused.