BibTeX:
@incollection{AbdulBah1886,
  author = {'Abdu'l-Bahá},
  title = {[Pages 21–40]},
  booktitle = {A Traveler's Narrative},
  year = {1886},
  url = {https://www.gutenberg.org/cache/epub/19300/pg19300-images.html},
}

APA:
'Abdu'l-Bahá. (1886). [Pages 21–40]. In *A Traveler's Narrative*.

MLA:
'Abdu'l-Bahá. "[Pages 21–40]." *A Traveler's Narrative*, 1886.

Chicago:
'Abdu'l-Bahá. "[Pages 21–40]." In *A Traveler's Narrative*, 1886.