Where does 'Hello world' come from?

Brian Kernighan actually wrote the first "hello, world" program as part of the documentation for the BCPL programming language developed by Martin Richards. BCPL was used while C was being developed at Bell Labs a few years before the publication of Kernighan and Ritchie's C book in 1972.

As part of the research for a book I was writing about the Alice programming environment, I corresponded with both Prof. Kernighan at Princeton and Martin Richards at Cambridge (when I was teaching a seminar there in the 1990’s). They helped me to track the first documented use of code to print the message "Hello, World!” Brian Kernighan remembered writing the code for part of the I/O section of the BCPL manual. Martin Richards -- who seems to have a treasure trove of notes, old documents, etc. -- found the manual and confirmed that this was the original appearance of the program. The code was used for early testing of the C compiler and made its way into Kernighan and Ritchie's book. Later, it was one of the first programs used to test Bjarne Stroustrup's C++ compiler.

It became a standard for new programmers after it appeared in Kernighan and Ritchie, which is probably the best selling introduction to programming of all time.

According to wikipedia:

While small test programs existed since the development of programmable computers, the tradition of using the phrase "Hello world!" as a test message was influenced by an example program in the seminal book The C Programming Language. The example program from that book prints "hello, world" (without capital letters or exclamation mark), and was inherited from a 1974 Bell Laboratories internal memorandum by Brian Kernighan, Programming in C: A Tutorial, which contains the first known version:

 main() {
        printf("hello, world");
 }

The first known instance of the usage of the words "hello" and "world" together in computer literature occurred earlier, in Kernighan's 1972 Tutorial Introduction to the Language B[1], with the following code:

main( ) {
  extrn a, b, c;
  putchar(a); putchar(b); putchar(c); putchar('!*n');
}
a 'hell';
b 'o, w';
c 'orld';

The first C program in the book "The C Programming Language" was to print "hello world!" on the screen.

Since then it is used as the first program to introduce the basic details of a programming language.