The Bab agreed the proposal of Baha'u'llah and appointed Mirza Yahya

Earlier, during the time that the Bab had been imprisoned at Chihriq, Baha’u’llah and one other Babi had suggested to the Bab that Mirza Yahya, Baha’u’llah’s young stepbrother, be known as the appointed nominee of the Bab until such time as ‘He Whom God shall make manifest’ would appear. The Bab had agreed to this proposal. Now, from Kirmanshah, Baha’u’llah sent Nabil-i-A‘zam to Tihran with instructions to escort Mirza Yahya, who had reached there from Mazindaran, away from the dangers of the capital. Baha’u’llah asked Nabil to remain with Mirza Yahya until such time as He could get back to Persia Himself.

(Day of Glory, The Life of Baha’u’llah, by Mary Perkins, George Ronald, p 32)