"Both the śrāvaka 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 anuttarasamyaksaṃbodhi. Anuttarasamyaksaṃbodhi is the realm of nirvana, which is the Dharma-body of the Tathāgata. " Srimaladevi Simhanada Sutra
《勝鬘師子吼一乘大方便方廣經》卷1 (CBETA, T12, no. 353, p. 220, c19-23)
"Therefore, if they turn away from the lesser vehicles, practitioners of the śrāvaka 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 amalavijñāna)."
A Discourse on the Srimaladevi Simhanada Sutra, Vol. 3, pp. 225-226