I couldn't find your response to this question. Are there quotes from the Scriptures of the other religions in Baha'u'llah's writings? Especially from Hindu and Buddhist Scripture? Then does Abdul Baha' have any quotes from them? And again, where in the Baha'i writings does it say that Baha'is should believe in all these "Holy Books"?
In various Tablets to the leaders of various religions, Baha’u’llah does quote His relevance to their Holy Books. For example in a Tablet to the Christians He refers to Himself as the ‘Spirit of Truth’, as the ‘Word’ that the disciples could not bear to hear at the time, and as the ‘Father’.
In Tablets to Muslims He refers to Himself as the ‘Great Announcement’ etc. To a Zoroastrian enquirer He confirmed He was the Shah Bahram foretold by Zoroaster. And similarly to Jews He stated He was the Promised One seated on the throne of David, the King of Glory.
Sorry but there are too many to list. You will have to research these for yourself so I have included some links for your convenience. There is also the Book of Certitude which explains the symbolical meaning behind many of the scriptures.
Baha’u’llah states in a Tablet that with interpretation not all are literal and not all symbolical. So as we consider Him to be the Promised One, we stand by His interpretations.
Bahá'í Reference Library - Proclamation of Bahá’u’lláh
Buddha, Krishna, Zoroaster and Related Subjects