The Bab appointed Mirza Yahya (known as Subh-i-Azal), to become the chief of the Babí community upon His passing.

Baha'i Scholar Peter Terry states,

The Báb appointed Mirzá Yahyá (known as Subh-i-Azal), to become the chief of the Bábí community upon His passing. Mirzá Yahyá assumed that position but did not exercise it in any systematic or constructive fashion, according to the Bahá'í reports of the period in question, from July 1850 until April 1863. Bahá'u'lláh became cognizant of His Mission, that He was "Him Whom God shall make manifest" promised by the Báb in many of His writings...

(Baha'u'llah In His Own Words by Peter Terry)