What follows is my provisional translation (in other words, not official or authorized; see here for more) of a Tablet from ‘Abduʼl-Bahá, the original text of which is published in Máʼidiy-i-Ásmání, vol. 2, pp. 50–52, available for download in DOC and PDF format.
In a Tablet to Ḥájí Ṣadr-i-Hamadání, the title of which is “O Ṣadruʼṣ-Ṣudúr! The unrolled scroll hath been read aloud,” the Center of the Covenant—exalted be His praise—hath said:
As to the question of the twelfth Imám, there existeth in the ḥadíths an apparent contradiction. In one ḥadíth, Muḥammad ibn Ḥasan —peace be upon Him—is considered the promised Qáʼim, yet in another, it is mentioned that the death of that same Muḥammad ibn Ḥasan will precede the birth of the awaited Qáʼim in the End of Time.  One cannot reconcile these two ḥadíths, unless he be certain that the twelfth Imám—the promised Qáʼim—existed spiritually in the Kingdom of God, and was manifested physically, in the form of another person, at the Hour of Revelation. “And no one thing is there, but with Us are its storehouses; and We send it not down but in settled measure” . That which is manifest in the contingent world existed first in the Kingdom of God.
The matter of the twelfth Imám and the promised Qáʼim is mentioned in a chain of highly questionable ḥadíths. Were one to judge the matter fairly, he would not give credence to any of these various narratives that contradict each other. Every one of the holy Imáms was considered by the Shíʻihs of His time to be the Qáʼim, and they awaited His emergence from the occultation.  With the passing of Imám Ḥasan ʻAskarí,  the leading divines realized that the foundation of hope for Shíʻihs would soon be utterly destroyed, and that they would fall into great despair. As these divines wished to preserve that hope, they made allusions, invoked metaphors, created allegories, and offered interpretations. Consequently, various narratives began to emerge, and the fact of the matter is that, after the passing of Imám Ḥasan ʻAskarí—peace be upon Him—the Shíʻihs had become divided into three groups. One group clung to the ignorant Jaʻfar,  and congratulated him on his accession to the Imámate. Another group came to renounce totally their belief in the role of the Imám. The other group attached themselves to belief in the occultation, and every day awaited the Qáʼim’s emergence therefrom. For a thousand years they have anticipated His advent, and to this day they have yet to grow tired of this futile anticipation.
Gracious God! The appearance of His Manifestation is mentioned explicitly in the Qurʼán, without any need for commentary or interpretation: “And thy Lord shall come and the angels rank on rank” . And yet have they regarded this matter with doubt, been unsettled by it, offered tenuous interpretations to explain it, and clung to narratives that have ultimately caused confusion and vacillation. They have deemed these narratives to be the explicit Text, and have used them to oppose anyone who should disagree with them. What foolishness, what folly have seized these people, and what stupidity they demonstrate! How utterly lost they are in their error! Salutations and praise be upon thee.
 The son of Imám Ḥasan ʻAskarí (the eleventh Imám of Shíʻih Islám), believed by Shíʻih Muslims to be the twelfth Imám.
 cf. Encyclopedia Iranica, “The Concept of Mahdi in Twelver Shiʿism”: http://www.iranicaonline.org/articles/islam-in-iran-vii-the-concept-of-mahdi-in-twelver-shiism
 Qurʼán 15:21.
 cf. Encyclopedia Iranica, “Ḡayba”: http://www.iranicaonline.org/articles/gayba
 The eleventh Imám of Shíʻih Islám.
 The brother of Imám Ḥasan ʻAskarí, known to Twelver Shíʻihs as “Jaʻfar-i-Kadhdháb” (“Jaʻfar the Liar”). For more information, refer to this Tablet from Baháʼuʼlláh: https://adibmasumian.com/translations/existence-of-qaim/
 Qurʼán 89:22.
A typed version of the complete Persian text of this tablet appears below.