The Kitáb-i-Aqdas - The Most Holy Book
Index term: Shrine of Bahá'u'lláh, under Holy Places and Historical Sites
n8.
and when the Sun of Truth and Utterance shall set, turn your faces towards the Spot that We have ordained for you
Bahá'u'lláh ordains His resting-place as the Qiblih after His passing. The Most Holy Tomb is at Bahjí, 'Akká. 'Abdu'l-Bahá describes that Spot as the "luminous Shrine", "the place around which circumambulate the Concourse on High".
In a letter written on his behalf, Shoghi Effendi uses the analogy of the plant turning in the direction of the sun to explain the spiritual significance of turning towards the Qiblih:

. . . just as the plant stretches out to the sunlight -- from which it receives life and growth -- so we turn our hearts to the Manifestation of God, Bahá'u'lláh, when we pray; . . . we turn our faces . . . to where His dust lies on this earth as a symbol of the inner act.