The first words ever spoken in a movie, "you ain’t heard nothin’ yet!"

Source

“Wait a minute… You ain’t heard nothin’ yet!” – The first words ever spoken in a movie, from The Jazz Singer (1927).
This improvised line by Al Jolson marked the end of silent films and the birth of the talkies.

Until this moment, films were entirely silent. The Jazz Singer wasn’t the first film to use sound, but it was the first to include synchronised spoken dialogue in a feature. That one line changed cinema forever.

Here's the clip:

A fascinating moment in film history.

Reference:
https://en.m.wikipedia.org/wiki/The_Jazz_Singer