The foundation of the Baha'i Community is rooted in those who came to the Baha'i Faith from Islam. Countless devout followers, even many noble Muslim clergymen became Baha'i's. Actually to say "convert" is not proper perse since Baha'u'llah's faith did not come to replace Islam, but to fulfill its destiny. Islam is also a general and a specific term (http://bahai-library.com/books/announcement.quran/4.html)
If you are looking for evidence as to why it is the fulfillment you should read the Kitab-i-Iqan, Revealed by Baha'u'llah.
http://www.amazon.com/Kitab-i-Iqan-Book-Certitude-Baha-llah/dp/1931847088
you can also read it online here, http://reference.bahai.org/en/t/b/KI/