What follows is my provisional translation (in other words, not official or authorized; see here for more) of a Tablet of Baháʼuʼlláh to Áqá ‘Abdu’r-Raḥím Bushrúyih’í. The original text has not yet been published; a copy of it was graciously supplied to me by Dr. Fridoon Rahimi, the grandson of Áqá ‘Abdu’r-Raḥím, and he informed me that he saw the original Tablet at the Bahá’í World Center. The copy he provided me with served as the basis for the Arabic typescript underneath the translation.
The following account of the iconic episode in Bahá’í history for which Áqá ‘Abdu’r-Raḥím is best known comes from Part 7 of The Blessed Beauty, a comprehensive and illustrated chronology of Bahá’u’lláh’s life prepared by Violetta Zein:
‘Abdu’r-Raḥím . . . traveled six months on foot to reach ‘Akká, arriving at the same time as Nabíl-i-A‘ẓam. ‘Abdu’r-Raḥím began circumambulating the Most Great Prison and saw Bahá’u’lláh’s hand beckoning him to enter the barracks. Raḥím entered through the gates unnoticed by the guards and soon entered the presence of Bahá’u’lláh, Who told him He had temporarily blinded the eyes of the guards so Raḥím might attain His presence. Raḥím stayed in the barracks for a few days, and left with several Tablets Bahá’u’lláh had revealed for him and Persian Bahá’ís.
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The Most Holy, the Most High
This is an epistle from Us unto him who attained to the Dawning-place of revelation and the Dayspring of lights, that he may so rejoice at the mention made by God that he shall not be saddened by the things which have befallen him.
Whensoever thou desirest the Visage of thy Lord—the Almighty, the Unconstrained—call thou to mind the time when thou didst enter the city as the prison gate was guarded by the hosts of iniquity, how We extinguished the brightness of their eyes and enabled thee to arrive at the Seat of the Throne with power and majesty, eventually appearing before Our Countenance, whereupon thou didst hear the voice of thy Lord, the All-Glorious, the All-Bountiful. Thine eyes were solaced by the presence of God and His beauty, and thou didst attain to that which most of the world’s peoples have been prevented from reaching. Indeed, thou didst quaff the wine of utterance from the mouth of the Most Merciful and drink from the limpid river of this Revelation, through which the faces of men have been made radiant and their eyes delighted. Well is it with him that hath set his heart toward the cynosure of lovers and attained the lights of the Countenance that illumineth all the world.
Then did Muḥammad-‘Alí circle round the Throne after We caused him to enter the prison. Thy Lord, in truth, ordaineth what He pleaseth. We permitted him to remain in the vicinity of his Lord for a few days as a token of Our grace. Verily, thy God is the Almighty, the Pardoner.
Give My remembrance to My loved ones, and recount to them what thou hast learned and seen. Thus biddeth thee thy Lord, the All-Glorious, the All-Knowing. We have conferred on thee Our name, “Raḥím”; strive that from thee may appear what We desire from this sublime title. When thou hast received this Tablet, place it upon thy face and say:
“Praised be Thou, O my God, for having enabled me to reach the Seat of the Throne of Thy grandeur and shone upon me with the effulgences of Thy Face. I beseech Thee by Thy Self to make me firm in Thy Cause and steadfast in Thy love. Thou, in truth, art the All-Powerful, the Almighty, the All-Compelling.”
 It is this very Tablet of Bahá’u’lláh to which Adib Taherzadeh refers where he writes, “It is not clear how many days ‘Abdu’r-Raḥím remained in the prison [of ‘Akká]. However, Bahá’u’lláh revealed a Tablet for him while he was there. In that Tablet He confirms that He had closed the eyes of the guards so that ‘Abdu’r-Raḥím could enter His presence and witness the glory of His countenance” (The Revelation of Bahá’u’lláh, vol. 3, p. 60).
 “In the days of Bahá’u’lláh, during the worst times in the Most Great Prison, they would not permit any of the friends either to leave the Fortress or to come in from the outside. ‘Skew-Cap’ [Mírzá Áqá Ján] and the Siyyid [Muḥammad Iṣfahání] lived by the second gate of the city, and watched there at all times, day and night. Whenever they spied a Bahá’í traveler they would hurry away to the Governor and tell him that the traveler was bringing in letters and would carry the answers back. The Governor would then arrest the traveler, seize his papers, jail him, and drive him out” (‘Abdu’l-Bahá, Memorials of the Faithful, no. 5).
 In the original Arabic, “Nabíl before ‘Alí,” a recasting of the name Muḥammad-‘Alí that features “Nabíl,” an Abjad equivalent of “Muḥammad” and a title given to this believer by Bahá’u’lláh: “Another eminent Bahá’í who came to ‘Akká but was recognized and expelled from the city was Mullá Muḥammad-‘Alí, surnamed Nabíl-i-Qá’iní [not to be confused with Nabíl-i-Akbar] by Bahá’u’lláh. He had known Bahá’u’lláh in Ṭihrán some years before the Báb declared His mission. At that time he had recognized the superhuman qualities of Bahá’u’lláh and become an ardent admirer of His person. When Nabíl-i-Akbar returned from ‘Iráq to his native town of Qá’in, he began to teach the Cause of the Báb openly to the public. As soon as Mullá Muḥammad-‘Alí heard the news of the Báb and Bahá’u’lláh, he became an ardent believer, saying that he had attained the presence of Bahá’u’lláh in Ṭihrán and had been deeply attracted to Him then. The next time he saw the face of His Lord was in the prison of ‘Akká. At first he was driven out of the city. But he made other attempts and eventually plans were made for him to enter the barracks. This he did successfully and was ushered into the presence of Bahá’u’lláh” (Adib Taherzadeh, The Revelation of Bahá’u’lláh, vol. 3, pp. 57–58).
 A title of God that translates to “the Merciful.”
A typescript of the original Arabic text of this Tablet appears below (all vocalization and punctuation mine).