The cppreference.com wiki says "1982: C with Classes reference manual published", but so far I haven't been able to track that down. Is that the correct title?
There are at least these two papers from that time period:
Stroustrup, Bjarne. "Classes: an abstract data type facility for the C language." ACM Sigplan Notices 17.1 (1982): 42-51.
Stroustrup, Bjarne. "Adding classes to the C language: An exercise in language evolution." Software: Practice and Experience 13.2 (1983): 139-161.