It is MOST important that security controls for a new system be documented in:

It is most important that security controls for a new system be documented in the system requirements. The system requirements define the functional and non-functional specifications of the system, including the security controls that are needed to protect the system and its data. Documenting the security controls in the system requirements can help ensure that they are designed, developed, tested, and implemented as part of the system development life cycle. Testing requirements, the implementation plan, and the security policy are other documents that may include security controls, but they are not as important as the system requirements. References = ISACA Certified in Risk and Information Systems Control (CRISC) Certification Exam Question and Answers, question 5; CRISC Review Manual, 6th Edition, page 212.