What establishes the truth of Baha’u’llah’s Revelation?

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I’m wondering how other people read this passage from Gleanings;

Say: The first and foremost testimony establishing His truth is His own Self.

Next to this testimony is His Revelation.

For whoso faileth to recognize either the one or the other He hath established the words He hath revealed
as proof of His reality and truth. –

Gleanings LII

It certainly leaves one wondering what then is His Revelation and where it is.