As far as I know, there are not many certifications, particularly for source code auditing. One certification that is relevant:
CISA: Certified Information Systems Auditor
Here are some other Certifications that would be helpful for this job role
- GIAC Software Security Certifications
- CISSP: Certified Information Systems Security Professional
- CSSLP: Certified Secure Software Lifecycle Professional
- CPT: Certified Penetration Tester
- CEPT: Certified Expert Penetration Tester
- GPEN: GIAC Certified Penetration Tester
- OSCP: Offensive Security Certified Professional
Due to the lack of certifications particular to this job role, if you are aiming to get a job in a specific company, research on which certifications that company expecting and then get into the program.