
...caused by relative motion between
the SOURCE of the wave and the observer.
The Big Bang Theory Season 1, Episode 6

So I went back to the SOURCE.
Game of Thrones Season 4, Episode 8

But not so deep as to introduce
a new SOURCE of friction."
The Big Bang Theory Season 2, Episode 5

I know that money can
be a SOURCE of friction
The Big Bang Theory Season 12, Episode 3

as a SOURCE of information
The Big Bang Theory Season 11, Episode 16