I'm fairly certain it was originally in BCPL.

You could just drop a note to Bjarne Stroustrup and ask. :-)

On page 44 of _The Design and Evolution of C++_ (Addison-Wesley, 1994), Stroustrup says:

“However, only C, Simula, Algol68, an in one case BCPL left noticeable traces in C++ as released in 1985. Simula gave classes, Algol68 operating overloading, references, and the ability to declare variables anywhere in a block, and BCPL gave // comments.”

He says a bit more about // comments on page 93, including an example of how they introduced an incompatibility with C.