Q. How do I cite something my professor said in class in APA Style?

APA 6th Edition

Since your instructor's comments aren't recoverable, please cite them as personal communication within the text e.g., (A. Surname, personal communication, September 4, 2018). See #8 in the "APA Style Citations Checklist" for information and examples. Remember that personal communication is not included in a reference list because the information isn't publicly accessible (American Psychological Association, 2010, p. 179).

Reference

American Psychological Association. (2010). Publication manual of the American Psychological Association (6th ed.). Washington, DC: Author. 

APA 7th Edition

Since your instructor's comments aren't recoverable, please cite them as personal communication within the text e.g., (A. Surname, personal communication, September 4, 2018). See #9 in the APA Style (7th Ed.) Citations Checklist for information and examples, and note that personal communication is not included in a reference list because the information isn't publicly accessible (American Psychological Association, 2020, p. 260).

Reference

American Psychological Association. (2020). Publication manual of the American Psychological Association (7th ed.). https://doi.org/10.1037/0000165-000