I think the clearest such passage might be this one:
The severe laws and injunctions revealed by the Báb can be properly appreciated and understood only when interpreted in the light of His own statements regarding the nature, purpose and character of His own Dispensation. As these statements clearly reveal, the Bábí Dispensation was essentially in the nature of a religious and indeed social revolution, and its duration had therefore to be short, but full of tragic events, of sweeping and drastic reforms. Those drastic measures enforced by the Báb and His followers were taken with the view of undermining the very foundations of Shí'ih orthodoxy, and thus paving the way for the coming of Bahá'u'lláh. To assert the independence of the new Dispensation, and to prepare also the ground for the approaching Revelation of Bahá'u'lláh, the Báb had therefore to reveal very severe laws, even though most of them were never enforced. But the mere fact that He revealed them was in itself a proof of the independent character of His Dispensation and was sufficient to create such widespread agitation, and excite such opposition on the part of the clergy that led them to cause His eventual martyrdom.
(On behalf of Shoghi Effendi, cited in Kitab-i-Aqdas, note 109
There are also such passages as these written by Shoghi Effendi himself:
"...this Book, of about eight thousand verses, occupying a pivotal position in Bábí literature, should be regarded primarily as a eulogy of the Promised One rather than a code of laws and ordinances designed to be a permanent guide to future generations."
(Shoghi Effendi, God Passes By, p. 25
"Designedly severe in the rules and regulations it imposed, revolutionizing in the principles it instilled, calculated to awaken from their age-long torpor the clergy and the people, and to administer a sudden and fatal blow to obsolete and corrupt institutions, it proclaimed, through its drastic provisions, the advent of the anticipated Day, the Day when "the Summoner shall summon to a stern business," when He will "demolish whatever hath been before Him, even as the Apostle of God demolished the ways of those that preceded Him." "
(ibid., p. 25
We perceive a no less apparent evolution in the scope of its teachings, at first designedly rigid, complex and severe, subsequently recast, expanded, and liberalized under the succeeding Dispensation, later expounded, reaffirmed and amplified by an appointed Interpreter, and lastly systematized and universally applied to both individuals and institutions.
(ibid., p. xvii
Some other passages of significance, relate to His Holiness, the Bab, Himself pointing out the fulfillment to come. For example:
"Today the Bayán is in the stage of seed; at the beginning of the manifestation of Him Whom God shall make manifest its ultimate perfection will become apparent."
(ibid., p. 30
There's also this passage which has a similar tone to the above ones, but it is about the conference of Badasht where Tahirih was to take off her veil.
The primary purpose of that gathering was to implement the revelation of the Bayán by a sudden, a complete and dramatic break with the past-- with its order, its ecclesiasticism, its traditions, and ceremonials.
(ibid., p. 31
Hope that helps...