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

Old age is a wonderful SOURCE of ironies,
Game of Thrones Season 4, Episode 9

...competed
over the same food SOURCE...
The Big Bang Theory Season 4, Episode 18

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

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