Format: 

In-text only: (A. A. Lastname, personal communication, Month, Day, Year)

Examples: 

In-text: (J. P. Smith, personal communication, July 31, 2015)

Notes: 

A personal communication can be private letters, memos, emails, interviews, telephone conversations, etc. Personal communications do not provide recoverable data and are not included in the reference list.

Source:
APA - 6.20