Naturalisation certificate reference number

Dear all,
While applying for a UK naturalisation i have been asked to provide my husband’s naturalisation certificate reference number and the date it was issued. My husband holds a british passport since 2021. He is not sure if he provided naturalisation certificate to home office when applied for his british passport and he does not even have any picture of the certificate to confirm the reference number. Any suggestion to what should we write in the application or how can we get the lost naturalisation certificate? Thankyou

He is not sure if he provided naturalisation certificate to home office when applied for his british passport

Well yes, he would have, it is compulsory. The certificate is what proves his British Citizenship, without which he wouldn't get a passport.

and he does not even have any picture of the certificate to confirm the reference number.

He should have the certificate. HM Passport Office would have sent it back to him with his foreign passport etc when he applied for his British passport.

Any suggestion to what should we write in the application or how can we get the lost naturalisation certificate?

If he does not have it anymore or has lost it, he will need to apply for a new one, costing £400.