The question sounds like a single-section lookup, but the recording-of-reasons requirement was written into the law in two separate places for two separate situations, and it helps to see why both exist before naming them.
The problem Parliament was solvingBefore 2010, an officer could arrest for any offence, however minor, without ever explaining on paper why that arrest was necessary. Custody itself became routine, used as a pressure tool rather than a step justified by the needs of investigation. The Code of Criminal Procedure (Amendment) Act, 2008 responded by rewriting Section 41 to force the officer to put a reason on record before taking that step, at least for offences carrying up to seven years.
Where the duty sits todayThat amendment is now Section 41(1)(b) of the CrPC, carried forward as Section 35(1)(b) of the BNSS. It requires the arresting officer to be satisfied, and to record in writing, that arrest is necessary for one of the listed purposes, such as preventing the person from committing a further offence, ensuring proper investigation, preventing tampering with evidence, or securing attendance in court.
The second half of the same ideaSection 41A of the CrPC, now Section 35A of the BNSS, deals with a different moment, after the police have already chosen to issue a notice of appearance instead of arresting. If the person does not comply with that notice, the officer may then arrest, but Section 41A(3) demands the same discipline: reasons for the arrest must be recorded before custody is taken.
Why the courts insisted on this being followedThe Supreme Court in Arnesh Kumar v. State of Bihar (2014) found that this statutory safeguard was being ignored in practice, particularly in matrimonial cruelty cases under Section 498A IPC, and directed police forces to actually comply with Section 41(1)(b) and 41A before making an arrest, on pain of departmental and contempt action.
So the answer has two limbs rather than one: Section 41(1)(b) CrPC, now Section 35(1)(b) BNSS, governs the decision to arrest without a warrant, and Section 41A(3) CrPC, now Section 35A BNSS, governs the decision to convert a notice of appearance into an arrest, and both require the reasons to be recorded in writing.