Mirza Yahya was the Bab's appointed trustee

No comment was made by Mirza Yahya Azal until later in Edime, where Baha’u’llah, in an epistle entitled Suriy-i-Amr, officially requested a statement from him as the Bab's appointed trustee. This document, written in Arabic in Baha’u’llah’s own hand, sets out clearly Baha’u’llah's claim to be the Man yuzhiruhu’llah foretold by the Bab and entrusted with a new revelation. This claim implied that a new dispensation had been initiated, that a new religion had come into being, and that the period of Mirza Yahya Azal’s trusteeship was over.

(Making the Crooked Straight: A Contribution to Bahá'í Apologetics, by Udo Schaefer, p. 612)