BibTeX:
@incollection{AbdulBah1909,
  author = {'Abdu'l-Bahá},
  title = {O Thou Who Art Firm in the Covenant!},
  booktitle = {Tablets of 'Abdu'l-Bahá Abbas},
  year = {1909},
  url = {https://www.gutenberg.org/cache/epub/19312/pg19312-images.html},
}

APA:
'Abdu'l-Bahá. (1909). O Thou Who Art Firm in the Covenant!. In *Tablets of 'Abdu'l-Bahá Abbas*.

MLA:
'Abdu'l-Bahá. "O Thou Who Art Firm in the Covenant!." *Tablets of 'Abdu'l-Bahá Abbas*, 1909.

Chicago:
'Abdu'l-Bahá. "O Thou Who Art Firm in the Covenant!." In *Tablets of 'Abdu'l-Bahá Abbas*, 1909.