> What work is this from?
It's from an excellent biography of Newton by Jonathan Aitken. Aitken was a British MP who served time in prison for perjury and corruption of justice charges. Just prior to his incarceration, he attended an 'Alpha Course' and was saved. Afterwards, he studied theology for the Church of England but refuses to be ordained because it would bring dishonor on the office of minister. So he wrote Newton's biography instead. It is very accessible, and written with wry English humor and understatement. It's not flashy and not hagiography. If you're interested in sketching a brief overview of Newton's life, I heartily recommend it.
John Newton: From Disgrace to Amazing Grace
Anyway, Aitken is no scholar, so he doesn't footnote every attribution of Newton's, but in the back of the book, he lists the sources he used in writing that particular chapter. It's in chapter 45 and the setup for it is:
>Newton’s ability to summarize the essentials of the gospel message in pithy phrases was often displayed in his pastoral letters. In writing to Mrs. Wathen (the wife of King George III’s oculist) in 1799, Newton set out his view of “the signs that accompany salvation”
And it lists some letter collections in the sources in the back.