Here are some samples of “Letters of Acceleration”.

Sample Demand Letter with notice to accelerate mortgage payments
Sample Demand Letter
Sample letter Notice of Intention to Foreclose from Bank of America.  Stating in default and that payments will be accelerated
Sample Bank of America – Notice of Intention to Foreclose
Sample letter to borrower stating account is in default and payments will be accelerated.
Sample letter of acceleration from America’s Servicing Co.