What is the knowledge referred to in the Hadith: "Whoever is asked about knowledge and conceals it will be bridled on the Day of Resurrection with a bridle of fire"—is it only religious knowledge, or does it include any type of knowledge?

The Hadith, "Whoever is asked about knowledge and conceals it will be bridled with a bridle of fire on the Day of Resurrection," pertains to the sciences of Sharia. The verse, "Indeed, those who conceal what We sent down of clear proofs and guidance..." is general and includes concealing rulings of the Sharia when there is a need for them. It is obligatory to teach necessary knowledge, such as informing a disbeliever about Islam or answering a questioner about what is lawful and unlawful, and whoever refrains from doing so is sinful. As for supererogatory knowledge (nawafil al-ilm) that people do not necessarily need to know, the matter is not the same.