声闻、缘觉乘皆入大乘，大乘者即是佛乘；是故三乘即是一乘，得一乘者得阿耨多罗三藐三菩提，阿耨多罗三藐三菩提者即是涅槃界，涅槃界者即是如来法身。
《胜鬘师子吼一乘大方便方广经》
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Both the ?rāvaka and pratyekabuddha vehicles are included in the Mahāyāna, which is the Buddha vehicle; therefore, all three are, in essence, one vehicle. Those who attain the one vehicle attain anuttarasamyaksa?bodhi. Anuttarasamyaksa?bodhi is the realm of nirvana, which is the Dharma-body of the Tathāgata.
The Lion’s Roar of Queen Srimala, the Single-Vehicle Great Expedience, the Corrective and Extensive Sutra
[Srimaladevi Simhanada Sutra]
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所以声闻乘、缘觉乘，如果回心以后一定是入大乘中，没有任何别的法是他们可以修学的；只有大乘法可以继续修学，而大乘法就是成佛的方法，所以大乘就是佛乘；除了大乘以外，没有其他的成佛之道。因此说，三乘之道其实就只有一乘道，而一乘道的法就是使大家修行之后，可以成就无上正等正觉的果证。
可是话说回来，无上正等正觉的境界是什么？所证的内涵又是什么？无非就是究竟涅槃的功德。而究竟涅槃的功德，其实就是如来的法身，那就是无垢识的境界。
《胜鬘经讲记》第三辑，页 225-226
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Therefore, if they turn away from the lesser vehicles, practitioners of the ?rāvaka and pratyekabuddha vehicles will invariably enter the Great Vehicle because there is no other Dharma for them to cultivate. Cultivation can only be continued in the Dharma of the Great Vehicle, which is the cultivation toward Buddhahood. Thus, the Great Vehicle is the Buddha vehicle, apart from which there is no other way to attain Buddhahood. Hence, it is said that the three vehicles are essentially one single vehicle, and the Dharma of this single vehicle enables Buddhist practitioners to attain the fruition of the supreme, complete, and perfect enlightenment.
However, what is the state of the supreme, complete, and perfect enlightenment and what are the contents realized in this state? They are none other than the attributes of the ultimate nirvana, which is the Dharma-body of the Tathāgata, and that is the state of the Immaculate Consciousness (the amalavij?āna).
A Discourse on the Srimaladevi Simhanada Sutra,Vol. 3, pp. 225-226