I did find the following from some searches on the Baha'i Academic Research Library:
Not only are the Imáms regarded as identical one with another,
[14] they are also identical in essence with the maj or prophet figures of the past: "I," says 'Alí in one tradition, "am Adam, I am Noah, I am Abraham, I am Moses,
I am Jesus, I am
Muhammad; I move through the forms as I wish — whoso has seen me has seen them, and whoso has seen them has seen me."
[55]
Here is footnote 55:
Hadith al-Sabába, quoted al-Ahsá'í, ibid., vol. 2, p. 54. Cf. ibid., p. 115, where the Imáms are said to have spread all the revealed religious systems (
shani'i')
. Al-Ahsá'i comments that, although the Imáms were created after Muhammad, they are like him in their essences. Ibid., vol. 4, p. 173)
Hierarchy, Authority and Eschatology in Early Bábí Thought
by Denis MacEoin
published in In Iran: Studies in Bábí and Bahá'í History vol. 3,
ed. Peter Smith (Los Angeles: Kalimat Press, 1986) pages 95-141
and
Thus hath Muhammad, the point of the Qur'án, revealed: "I am all the Prophets." Likewise, He saith: "I am the first Adam, Noah, Moses, and Jesus." . . . Wherefore, should one of these Manifestations of Holiness proclaim saying: "I am the return of all the Prophets," He verily speaketh the truth. In like manner, in every subsequent Revelation, the return of the former Revelation is a fact, the truth of which is firmly established.
KI, pp. 152-154, italics added. 'Abdu'l-Bahá is recorded to have said "[T]he Bab states, 'I am the return of all the Prophets.'" PUP, p. 16
So it is a tradition to be sure and most probably a Shiah one attributed to Prophet Muhammad ... there are so many of these that they are still not readily avaialble to most people. But for us Baha'is...if Baha'u'llah cites it we accept it regardless. It would be problematic to cite it in a Muslim forum I would think as many of them probably wouldn't accept the Shiah view anyway...
- Art