form 15 CA & 15 CB

Yes, Authorized dealer banks have to ensure that the applicable taxes have been paid on the income/funds that you intend to remit/repatriate abroad. Hence, while remitting funds from NRO account, you need a certificate from chartered account (Form 15CB) and an undertaking (Form 15CA) along with the remittance request form. The remitting bank may ask for additional documents based on the source of funds you seek to remit.

Refer:   nri-corner-remittance-facilities-for-nri