The Kitáb-i-Aqdas - The Most Holy Book
Index term: Baghdád, under Spot, in reference to
k133.
Raise up and exalt the two Houses in the Twin Hallowed Spots, and the other sites wherein the throne of your Lord, the All-Merciful, hath been established. Thus commandeth you the Lord of every understanding heart.
n154.
Raise up and exalt the two Houses in the Twin Hallowed Spots, and the other sites wherein the throne of your Lord . . . hath been established.
Bahá'u'lláh identifies the "two Houses" as His House in Baghdád, designated by Him as the "Most Great House", and the House of the Báb in Shíráz, both of which have been ordained by Him as sites of pilgrimage. (See Q&A 29, 32 and note 54.)
Shoghi Effendi explained that "the other sites wherein the throne of your Lord . . . hath been established" refers to those places where the Person of the Manifestation of God has resided. Bahá'u'lláh states that "the people of the areas where these are situated may choose to preserve either each house" wherein He resided, "or one of them" (Q&A 32). Bahá'í institutions have identified, documented, and where possible, acquired and restored a number of the historical sites associated with the Twin Manifestations.
q32.
 
Question: Concerning the restoration and preservation of the two Houses in the Twin Spots, and the other sites wherein the throne hath been established.
Answer: By the two Houses is intended the Most Great House and the House of the Primal Point. As for other sites, the people of the areas where these are situated may choose to preserve either each house wherein the throne hath been established, or one of them.