Explanation of the idea that we were all present at sinai when the torah was given?

Hi y’all, i was wondering about this. I’ve heard it said that all Jews living today were present at Mt Sinai when hashem gave us the Torah, usually in connection with converts.

Where does this come from? Like who formulated the idea or what book does it come from?

Edit: is this also the mainstream belief overall? (And if it isn’t what is?)