This safeguard comes from Article 22(1) of the Constitution, and the reason it exists is more useful to remember than the bare citation.
An arrested person who does not know why they have been arrested cannot do anything to defend themselves, cannot instruct a lawyer on the actual facts, and cannot ask a court for release through a habeas corpus petition, because such a petition needs something specific to challenge. Article 22(1) closes that gap by requiring that grounds be communicated to the arrested person as soon as may be.
Over time courts have tightened what being informed means. It is not satisfied by a vague oral remark at the time of arrest. It must be a real, specific, and effective communication, and in cases involving special laws such as UAPA, the Supreme Court in Prabir Purkayastha v. State (2024) has held it must be given in writing.
So the constitutional basis is Article 22(1), and its practical content today is a written, specific statement of why the person has been arrested.
Naming Article 22(1) is the easy part of this question; understanding how demanding that Article has become is the part worth walking through, because the standard has moved considerably over the decades.
The starting pointEvery one of these decisions is an application of the same constitutional guarantee, Article 22(1)'s promise that no arrested person is kept in custody without being informed of the grounds for arrest, and the case law simply shows that promise being read more strictly with each passing decade.
\[ \boxed{\text{Article 22(1) of the Constitution of India is the source of this requirement.}} \]