Could you please provide some provenance for that story? I regret to say that I don't entirely trust its accuracy -- it sounds, how shall I say it, apocryphal.
It comes from the recordings of the stories told about Baha'u'llah by his son Abdul'baha, who shared his excile and imprisonment. This was during the time in Baghdád.
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This is that record.
Note at the end that this story will have other sources.
"It often happened in Ba
ghdád that Muslim, Jewish, and Christian divines and European men of learning would be gathered in His blessed presence. They would each ask a different question and, despite their varying beliefs, would each receive so complete and convincing a reply as to be fully satisfied. Even the Persian divines residing in Karbilá and Najaf chose a learned man by the name of Mullá Ḥasan ‘Amú and dispatched him as their representative. He came into His blessed presence and asked a number of questions on their behalf, to which Bahá’u’lláh responded. He then said, “The divines fully recognize the extent of your knowledge and attainments, and it is acknowledged by all that you are without peer or equal in every field of learning. It is moreover evident that you have never studied or acquired this learning. But the divines say that they are not satisfied with this and cannot acknowledge the truth of your claim on the basis of your knowledge and attainments alone. They therefore ask you to produce a miracle in order to satisfy and assure their hearts.”
Bahá’u’lláh replied, “Although they have no right to ask this, since it is for God to test His creatures and not for them to test God, yet their request is in this case accepted and allowed. But the Cause of God is not a theatrical stage where every hour a new performance may be offered and every day a new demand presented. For otherwise the Cause of God would become the plaything of children.
“Let the divines, therefore, assemble and choose unanimously one miracle, and let them stipulate in writing that once it has been performed they will no longer entertain any doubt, but will all acknowledge and confess the truth of this Cause. Let them seal that paper and bring it to Me. They must fix this as the criterion of truth: If it be performed, they should have no remaining doubt; and if not, We shall stand convicted of imposture.”
That learned man arose and replied, “There is no more to be said.” He kissed Bahá’u’lláh’s knee, even though he was not a believer, and departed. Then he gathered the divines and conveyed Bahá’u’lláh’s message. They consulted together and said, “This man is a magician; perchance he will perform some enchantment, and then we will have no recourse”, and so they dared not respond.
Mullá Ḥasan ‘Amú, however, reported this fact in many gatherings. He left Karbilá for Kirmán
sháh and Ṭihrán, where he provided all with a detailed account of this episode and spoke of the fear and inaction of the divines."
Regards Tony