I just googled part of your quote, and it appeared on at least this tombstone, but there's no reference to the original:

In memory of Mrs. Abigail, wife of Capt. Josiah Snell; she died Dec. ye 2, 1784, in ye 76 year of her age.

Remember me as you pass by,
For as you are so once was I;
As I am now so you must be;
Prepare for death and follow me.

If you google part of it, you'll end up with about four pages (not too many, all things considered, to search further).