A number of online sources (such as this article by Rabbi Ari Enkin, or this article by Rabbi Yair Hoffman) mention the tradition that rabbonim used to go out into the woods and teach Torah while pretending to do something that the Romans considered innocuous (like playing with bows and arrows).
So far, I have yet to find an article that mentions a source for this claim.
What is the earliest source that rabbonim used to teach Torah in secret, out in the woods, using a ruse of this nature?