APA Citation Style (7th Edition)

General format for citing case studies:
Author(s). (Year). Title of case study. Number of case study. URL.

Examples:
Harvard Business School Case Study
Smith, S. (2003). Leadership. HBS No. 7-806-122. https://hbsp.harvard.edu/cases/
Ivey Business School Case Study
Heisz, M., & Leech, L. (2005). Sarbanes-Oxley Act of 2002: Understanding the requirements and the Canadian response. Ivey ID: 9B05B009. https://www.iveycases.com/

(Examples courtesy of APA Style Guidelines & Examples - NAIT Library.)

Textbook Case Study
Author(s) or editor(s) of the chapter or entry or case study. (Year of book). Title of chapter or entry or case study. In First initial. Last name, & First initial. Last name (Eds.), Title of book (pp. xx-xx). Publisher.
Example:
Pederson, S. (2008). The XYZ Group. In J. Ness, Cases in digital processing (pp.11-20). CDMA Publishing.

An online retrieval date is only required for an unarchived source that is likely to change, such as a Twitter profile or Facebook page, or a webpage with no date. 

(See section 9.16 of the APA Publication Manual, 7th ed.)