This is of course the famous end quote from Casablanca, spoken by Humphrey Bogart's Rick to Captain Renault played by Claude Rains.

There's a lot said about Casablanca in the terrific node on the subject, but it fails to mention some interesting things about the end of the movie, and this line in particular. When the production of the movie started, the script and screenplay - which was written by Julius& Philip Epstein - weren't finished yet. Director Michael Curtiz wouldn't and actually couldn't let Ingrid Bergman know whether her Ilsa would finally end up with Rick or Victor Laszlo. He simply told her to "play it in between". Some claim that this has added to the magic that surrounds this movie.

In the first version, Ilsa would end up with Laszlo and fly away from Rick; in the alternate and more expected and conventional ending, she would stay behind with Rick. When the film studio had seen the first ending, they did what Hollywood all too seldom does today; letting Rick lose Ilsa. Maybe this is why the movie feels so fresh even on our time. The second ending was never even shot. First three weeks after the production was finished, Humphrey Bogart was asked to record a final line for the last scene of the movie.

As Rick and Captain Renault are seen from behind, slowly walking across the runway where Ilsa's plane just have left, into the darkness and enveloping fog, Rick says:

"Louis, I think this is the beginning of a beautiful friendship."