The Kitáb-i-Aqdas - The Most Holy Book
Index term: Bahjí­
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.
n54.
The Lord hath ordained that those of you who are able shall make pilgrimage to the sacred House
Two sacred Houses are covered by this ordinance, the House of the Báb in Shí­ráz and the House of Bahá’u’lláh in Baghdád. Bahá’u’lláh has specified that pilgrimage to either of these two Houses fulfils the requirement of this passage (Q&A 25, 29). In two separate Tablets, known as Súriy-i-Hajj (Q&A 10), Bahá’u’lláh has prescribed specific rites for each of these pilgrimages. In this sense, the performance of a pilgrimage is more than simply visiting these two Houses.
After the passing of Bahá’u’lláh, ‘Abdu’l-Bahá designated the Shrine of Bahá’u’lláh at Bahjí­ as a place of pilgrimage. In a Tablet, He indicates that the “Most Holy Shrine, the Blessed House in Baghdád and the venerated House of the Báb in Shí­ráz” are “consecrated to pilgrimage”, and that it is “obligatory” to visit these places “if one can afford it and is able to do so, and if no obstacle stands in one’s way”. No rites have been prescribed for pilgrimage to the Most Holy Shrine.