What follows is my provisional translation (in other words, not official or authorized; see here for more) of a Tablet from Baháʼuʼlláh, which can be found in Máʼidiy-i-Ásmání, vol. 1, p. 7. (A scan from this collection is available online here.)
Regarding the one who related the existence of the Qáʼim 
In a Tablet to Mírzá Áqáy-i Afnán, the blessed pen of the Ancient Beauty—exalted be His glory—hath revealed:
“The one who related the existence of the Qá’im was a woman. It was asked of Jaʻfar,  ‘Does [Imám Ḥasan] ʻAskarí have any living sons?’ That wronged one  declined and said, ‘There was a child, but he passed away two years ago.’ Those with ill intentions condemned him as a liar, and they shunned and cursed him. Ignorant and heedless, they accepted and proclaimed as truth the words of that lying woman, which were in agreement with their selfish intentions.”
 According to the Twelver Shíʻih tradition, the twelfth Imám—known as the Mahdí, or “rightly guided one”—was born to the eleventh Imám, Ḥasan ʻAskarí, in 869 AD. After Ḥasan ʻAskarí died in 874, the Mahdí—who would have been five years old at the time—was purportedly hidden to protect him from the ruling ʻAbbásid caliphate. He would communicate with his followers through a series of four intermediaries. This period, known as the Minor Occultation, ended in 941 with the death of the fourth intermediary. Since that time, Twelver Shíʻihs believe that the Mahdí has continued to exist in hiding, and will eventually reveal himself to battle the forces of evil and firmly establish submission unto God—the literal definition of “Islám”—as the universal religion for mankind.
Moojan Momen refers to this Tablet in his article, “Shi`i Islam” (1995; available online), as evidence that Baha’u’llah rejected the story of the twelfth Imám as a pious fraud.
For more information on the concept of the Mahdí, refer to the following articles in the 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
- “Ḡayba”: http://www.iranicaonline.org/articles/gayba
 The brother of Imám Ḥasan ʻAskarí, known to Twelver Shíʻihs as “Jaʻfar-i-Kadhdháb” (“Jaʻfar the Liar”). In this tablet, Baha’u’llah dismisses this tenet of Shíʻih eschatology by establishing the truth of Jaʻfar’s reply to the question about his brother’s son. (This is not the same person as Imám Jaʻfar aṣ-Ṣádiq, who lived in the eighth century AD—roughly a hundred years before Jaʻfar-i-Kadhdháb.)
 This characterization of Jaʻfar is likely a reference to the fact that generations of Twelver Shíʻihs have considered him a liar, even though he was actually telling the truth.
A typed version of the complete Persian text of this Tablet appears below.