The source is from the Baha'i Writings.
"..Thus hath Muḥammad, the Point of the Qur’án, revealed:
“I am all the Prophets.” Likewise, He saith: “I am the first Adam, Noah, Moses, and Jesus.” Similar statements have been made by Imám ‘Alí. Sayings such as these, which indicate the essential unity of those Exponents of Oneness, have also emanated from the Channels of God’s immortal utterance, and the Treasuries of the gems of Divine knowledge, and have been recorded in the Scriptures...."
Also in the Kitab-i-iqan pages 162-164
" ...how many are those who, through failure to understand its meaning, have allowed the term "Seal of the Prophets" to obscure their understanding, and deprive them of the grace of all His manifold bounties! Hath not Muhammad, Himself, declared: "I am all the Prophets?" Hath He not said as We have already mentioned: "I am Adam, Noah, Moses, and Jesus?" Why should Muhammad, that immortal Beauty, Who hath said: "I am the first Adam" be incapable of saying also: "I am the last Adam"? For even as He regarded Himself to be the "First of the Prophets" - that is Adam - in like manner, the "Seal of the Prophets" is also applicable unto that Divine Beauty. It is admittedly obvious that being the "First of the Prophets," He likewise is their "Seal."
This topic requires an extensive exploration of Hadith.
Regards Tony