Marshall Mcluhan used the phrase ‘the medium is the message’ in his book Understanding Media: The Extensions of Man (1964). In essence, it suggests that the medium itself symbiotically embeds meaning in a message, and affects how it is perceived.

This video shows Marshall Mcluhan discussing the notion of the media being the message in 1977:

Lecture recorded by ABC Radio National Network on 27 June 1977 in Australia. (Mcluhan, 1977)