The person making the arrest must inform the person to be arrested of the intention to arrest him, of the cause of the arrest, and the authority to make it, except when the person making the arrest has reasonable cause to believe that the person to be arrested is actually engaged in the commission of or an attempt to commit an offense, or the person to be arrested is pursued immediately after its commission, or after an escape.
The person making the arrest must, on request of the person he is arresting, inform the latter of the offense for which he is being arrested.
Ca. Pen. Code § 841