OOh, lovely. It's from the movie 'Holiday INN'. Wiki says:
Following its prominence in in the musical Holiday Inn, the composition won the Academy Award for Best Original Song. In the film, Bing Crosby sings "White Christmas" as a duet with actress Marjorie Reynolds, though her voice was dubbed by Martha Mears. This now-familiar scene was not the moviemakers' initial plan; in the script as originally conceived, Reynolds, not Crosby, was to sing the song.[2]