You must submit one or a few sample chapters with a nonfiction book proposal.Concerning what chapter (s) to submit, do not submit the introduction if you are only submitting one sample chapter. (twliterary.com)
All you need is a book proposal and a few sample chapters. (query-letter.com)
It's similar to when you buy a print book and there are a few sample chapters in the back, except it's in digital format and available to everyone. (ingramspark.com)