The Kitáb-i-Aqdas - The Most Holy Book
Index term: Sun of, under Truth
k6.
n3, n4, n5, n6, n7, n8
We have enjoined obligatory prayer upon you, with nine rak'ahs, to be offered at noon and in the morning and the evening unto God, the Revealer of Verses. We have relieved you of a greater number, as a command in the Book of God. He, verily, is the Ordainer, the Omnipotent, the Unrestrained. When ye desire to perform this prayer, turn ye towards the Court of My Most Holy Presence, this Hallowed Spot that God hath made the Centre round which circle the Concourse on High, and which He hath decreed to be the Point of Adoration for the denizens of the Cities of Eternity, and the Source of Command unto all that are in heaven and on earth; and when the Sun of Truth and Utterance shall set, turn your faces towards the Spot that We have ordained for you. He, verily, is Almighty and Omniscient.
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.