There is a well known aggadata from maseches Brachos 54b concerning Og desiring to throw a mountain on klal yisroel and how Hashem arranged his demise.
However, I was reading to one of my children from 'Tell me the story of the parsha'. In parshas Devarim on page 19 is the following:
Next to the might Og the Yidden looked like ants. He lifted up a big, heavy mountain and threw it at the Yidden, sure that it would kill them all at once. But Moshe Rabbeinu quickly wrote the name of Hashem on his stick and picked up the mountain that Og had thrown.
Afterwards the book continues with the aforementioned, well known aggadata.
There are many embellishments throughout this book. But usually the stories are based off some basic source. I don't know of one for this story. I checked in my Otzar Ishei Tanach and didn't find anything close to this, only the other aggadata.
So, is there a source for this story?