Baha'u'llah claims that he is the return of Imam Husayn

In Addressing himself more directly to the Ithna ‘Ashari Shi’i sect which is predominant in his native Iran, Baha’u’llah writes in "Lawh-i-Husayn":

GL:12: Consider the eagerness with which certain peoples and nations have anticipated the return of the Imam Husayn, whose coming, after the appearance of the Qa'im, hath been prophesied, m days past, by the chosen ones of God, exalted be His glory. These holy ones have, moreover, announced that when He Who is the Day Spring of the manifold grace of God manifesteth Himself, all the Prophets and Messengers, including the Qa’im, will gather together beneath the shadow of the sacred Standard which the Promised One will raise. That hour is now come. The world is illumined with the effulgent glory of His countenance.

Proofs of the Prophets--The Case for Baha'u'llah By Peter Terry