Bahá'u'lláh acknowledges that the Báb is “the Primal Point, the Divine Mystery, the Unseen Essence, the Day-Spring of Divinity”, and the “Manifestation” of God’s “Lordship”

Magnify Thou, O Lord my God, Him Who is the Primal Point, the Divine Mystery, the Unseen Essence, the Day-Spring of Divinity, and the Manifestation of Thy Lordship, through Whom all the knowledge of the past and all the knowledge of the future were made plain, through Whom the pearls of Thy hidden wisdom were uncovered, and the mystery of Thy treasured name disclosed, Whom Thou hast appointed as the Announcer of the One through Whose name the letter B and the letter E have been joined and united, through Whom Thy majesty, Thy sovereignty and Thy might were made known, through Whom Thy words have been sent down, and Thy laws set forth with clearness, and Thy signs spread abroad, and Thy Word established, through Whom the hearts of Thy chosen ones were laid bare, and all that were in the heavens and all that were on the earth were gathered together, Whom Thou hast called Ali-Muhammad in the kingdom of Thy names, and the Spirit of Spirits in the Tablets of Thine irrevocable decree, Whom Thou hast invested with Thine own title, unto Whose name all other names have, at Thy bidding and through the power of Thy might, been made to return, and in Whom Thou hast caused all Thine attributes and titles to attain their final consummation. To Him also belong such names as lay hid within Thy stainless tabernacles, in Thine invisible world and Thy sanctified cities.

Impact on the family in Tehran

The house of Nuri relatives in Tehran must have buzzed with the realization of the dramatic injection into their family of a potent new religious message now supported by the family leader and genius. Not surprising then that a half-brother, Mirza Yahya, fourteen at that moment of Mulla Husayn's mission, should also have become a Bábi, following the lead of his older half-brothers. This youth had been taken under the special tutorship of Bahá'u'lláh, becoming His ward upon the early passing of his mother Kuchik, and already had been under His brother's instruction from his early years. He was grave, silent, courteous, modest, a solitary who was fond of the poems of the mystics and was later termed 'an amiable child ' by an older Babi. And perhaps there were even then others of the family also willing quietly to follow the dangerous, unconservative Bábi ideas which were sure to lead to trouble with the mullas. Ultimately, in addition to Mirza Musa, three other brothers and several sisters espoused the new Faith. 
(Adapted from ‘Robe of Light, vol. 1’, by David Ruhe)

Bahá'u'lláh acknowledges that the Báb “hath been sent down by the Will of the one true God”

We, verily, believe in Him Who, in the person of the Báb, hath been sent down by the Will of the one true God, the King of Kings, the All-Praised. We, moreover, swear fealty to the One Who, in the time of Mustaghath, [1] is destined to be made manifest, as well as to those Who shall come after Him till the end that hath no end. We recognize in the manifestation of each one of them, whether outwardly or inwardly, the manifestation of none but God Himself, if ye be of those that comprehend. Every one of them is a mirror of God, reflecting naught else but His Self, His Beauty, His Might and Glory, if ye will understand. All else besides them are to be regarded as mirrors capable of reflecting the glory of these Manifestations Who are themselves the Primary Mirrors of the Divine Being, if ye be not devoid of understanding. No one hath ever escaped them, neither are they to be hindered from achieving their purpose. These Mirrors will everlastingly succeed each other, and will continue to reflect the light of the Ancient of Days. They that reflect their glory will, in like manner, continue to exist for evermore, for the Grace of God can never cease from flowing. This is a truth that none can disprove. 

- Baha'u'llah  (‘Gleanings from the Writings of Baha'u'llah’)

[1] Literally, He Who is Invoked for Help. A term the Báb used to refer to the advent of the Promised One (Baha’u’llah) (Footnote in ‘Call to Remembrance, p. 23)

Bahá'u'lláh instantly accepts the Báb’s spiritual authority

Bahá'u'lláh’s brother, Mirza Musa and Mulla Husayn’s messenger were both enraptured by listening to the sound of Bahá'u'lláh’s voice and the sweetness of its melodies as He recited a single section of the scroll that was presented to Him – a scroll from the Báb containing some passages from of the Báb’s Writings, the Qayyumu’l-Asma. They were startled when heard Him say to His brother:

“Musa, what have you to say? Verily I say whoso believes in the Qur’an and recognizes its divine origin, and yet hesitates, though it be fora moment, to admit that these soul-stirring words are endowed with the same regenerating power, has most assuredly erred in his judgment and strayed far from the path of justice” (‘The Dawn-Breakers’)

To that scroll fragment of the Qayyumu’l-Asma the hypersensitive Mirza Husayn-'Ali [Bahá'u'lláh] had responded as a chord plucked by the Divine Harpist. Cursorily acquainted as he probably was with the preparatory doctrines of Shaykh Ahmad and Siyyid Kazimim, His immediate recognition of the Bab's spiritual authority aligned Him with the new Cause. It was as though He were always a Bábi, so ready of spirit was He. Indeed, His inherent readiness for the new Truth assured instant recognition of the Báb, and thus He became another primary Disciple of the new High Prophet. In that moment, too, He began His own Mission from God.

Bahá'u'lláh’s perceptive brother Musa, also upon hearing the reading of a single long passage, almost simultaneously identified with this newborn Truth. 
(Adapted from ‘Robe of Light, vol. 1’, by David Ruhe)

The Báb’s messenger finds Bahá'u'lláh in Tihran and delivers the Message

In accordance with the Báb’s singular instructions of great promise, and carrying precious scrolls, Mulla Husayn set off on this first mission for his Master with his characteristically high spirit and persuasiveness, seeking out the Shaykhis en route. Isfahan, Kashan and Qum were essentially obdurate: in Isfahan he found only a youth, a sifter of wheat, and some five others destined for great service in the new Cause; in Kashan a single to-be-distinguished convert; in fanatical Qum none. But there was much planting of seeds along his route through his challenging words on the new Teachings.

Arriving in Tihran in late July or early August to discover the “Mystery of transcendent holiness” that the Báb had promised him, Mulla Husayn settled into a room in a religious college not far from the Shimiran Gate where Baha’u’llah’s rented house was located. There Haji Mirza Muhammad of Khurasan, leader of the Shaykhis of Teheran, welcomed his distinguished associate, but was skeptical of the new Teachings which contained such disturbingly convincing contents and were presented so vehemently. He even went so far as to rebuke Mulla Husayn for his failure to fortify the Shaykhi community in its doctrines. But to a young Shaykhi, a favored disciple of the leader and a priest-tutor, Mulla Muhammad Mu'allim [teacher] of Nur district, fell the signal honor of becoming the courier of Mulla Husayn.

The youth's room in the college was adjacent to that of his master, and he overheard the equivocal conversation with Mulla Husayn. Impressed by the great powers and passion of the visitor, he felt keenly the unsatisfactory behavior and cautious attitude of his master; moreover, he was attracted by Mulla Husayn’s vibrant personality. Awaiting an opportunity for private discussion, he knocked on Mulla Husayn's door that midnight, found him awake and seated beside his lamp. He was welcomed with such heartfelt warmth that he was impelled to discharge his strong feelings.

The First Letter of the Living is given the assignment to bring the Message of the Báb to Bahá'u'lláh

Once eighteen believers had recognized the Báb, each independently of the others, the Báb called them the ‘Letters of the Living’, and one by one sent them “with steadfast feet and sanctified hearts” on perilous specific tasks to proclaim the new Message.

To Mulla Husayn, His first Letter of the Living, the Báb pointed to the north, saying: “Grieve not that you have not been chosen to accompany Me on My pilgrimage to Hijaz. I shall instead direct your steps to that city [Tihran] which enshrines a Mystery of such transcendent holiness as neither Hijaz [a region in Saudi Arabia containing the holy cities of Mecca and Medina] nor Shiraz can hope to rival… Visit, on your way, Isfahan, Kashan, Tihran and Khurasan… The hosts of the invisible Kingdom, be assured, will sustain and reinforce your efforts. The essence of power is now dwelling in you, and the company of His chosen angels revolves around you. His almighty arms will surround you, and His unfailing Spirit will ever continue to guide your steps…” (‘The Dawn-Breakers’)

Of his mission to Tihran, the Báb presented an exciting prospect, saying: “A secret lies hidden in that city. When made manifest, it shall turn the earth into paradise. My hope is that you may partake of its grace and recognize its splendor.” (‘The Dawn-Breakers’) 

(Adapted from ‘Robe of Light, vol. 1’, by David Ruhe)