1. ISO/IEC 19011:2018, Guidelines for auditing management systems.
Section 3.1, Note 3 to entry: This standard explicitly defines the types of audits. It states, "Internal audits, sometimes called first-party audits, are conducted by, or on behalf of, the organization itself... Third-party audits are conducted by independent auditing organizations, such as those providing certification/registration of conformity to the requirements of ISO 9001 or ISO 14001." (This principle applies directly to ISO 27001 certification audits).
2. ISO/IEC 27001:2022, Information security, cybersecurity and privacy protection — Information security management systems — Requirements.
Clause 9.2.1 General: This clause mandates that "The organization shall conduct internal audits at planned intervals..." This supports the first part of Statement 1 but does not specify the frequency of external audits.
3. Calder, A., & Watkins, S. G. (2019). IT Governance: An International Guide to Data Security and ISO 27001/ISO 27002 (7th ed.). Kogan Page Publishers.
Chapter 14, "Certification and continual improvement," p. 188: This chapter details the certification process, explaining, "Certification is for a fixed period, usually three years, at the end of which you will need to apply for recertification... In the intervening years, your certification body will carry out periodic surveillance audits to ensure that you are continuing to maintain the ISMS." This confirms that the certification audit is not a single annual event.