“The gravest offender among the Muslims is the one who asks about something that was not forbidden, and then it was made forbidden because of his questioning.”

Narrated by al-Bukhārī (no. 7289, and this is his wording) and Muslim (no. 2358), from the ḥadīth of Saʿd ibn Abī Waqqāṣ (may Allah be pleased with him).

Brief Explanation of the Hadith

This ḥadīth points to a great educational principle regarding asking questions and issuing fatwas: the foundation of Islamic law is to remove hardship from those who are accountable and not to impose unnecessary restrictions. What the Sharīʿah has remained silent about is out of mercy, not out of forgetfulness, and therefore should not be asked about out of stubbornness or needless insistence. Hence, the Prophet — may Allah’s peace and blessings be upon him — warned against asking out of stubbornness or idle curiosity, lest such questioning lead to something being imposed on the Ummah. For an excessive question may open the door to stricter rulings, so that what was once permissible becomes forbidden. In that case, the one who asked becomes the cause of depriving himself and others of what was allowed, and thus deserves the description of being among the gravest in sin from this perspective.
And the meaning of “crime” (jurm) here is: a great sin. An example of this is what happened to the Children of Israel in the story of the cow.
Likewise, the saying of the Prophet — may Allah’s peace and blessings be upon him — when pilgrimage was made obligatory: “If I had said: ‘Yes,’ it would have become obligatory, and you would not have been able to do it… Leave me so long as I leave you; for those before you were only destroyed because of their excessive questioning and their disagreements with their prophets.”
And what is meant by this warning is not the prohibition of asking for the sake of learning and seeking clarification. Rather, one must ask about matters that are necessary to know; for this is commanded in Allah’s saying: {So ask the people of knowledge if you do not know} (al-Naḥl: 43).
This warning was specific to the time of the Prophet — may Allah’s peace and blessings be upon him —, out of fear that something might be made obligatory upon his Ummah which would be difficult for them. As for after the cessation of revelation, the principle is to ask the scholars about what is unclear, since the Islamic Sharīʿah has been completed and nothing can be added to it or taken away from it.
In this ḥadīth: the sin is graver when its harm is widespread, and one of the major legal principles is that the basic ruling on things is permissibility until there is evidence from the Sharīʿah to the contrary.
It also shows the Prophet’s ﷺ mercy and compassion toward his Ummah, and his fear that something might be made obligatory upon them which they would be unable to bear.