Source of quote about the hardships of book writing: "... It was a good day."

I once heard a book author being interviewed. He was explaining how difficult and painful it was for most authors to write. It is a hardship that only fellow writers seem to appreciate, he/she said. To illustrate, he quoted a letter that one (famous?) author wrote to another. It went something like this:

“Wrote 100 words on Tuesday. It was a good day.”

Anyone know the exact quote and the author?

Since everyone’s stumped, this thread should be bumped!