大乘起信論別記 1.195

1.195

言「於色究竟處示高大身」者，如十卷經偈云「欲界及無色，佛不彼成佛；色界中上天，離欲成菩提。」今釋此經意云「若論實受用身之義，遍於法界無處不在，而言唯在彼天之身而成佛者，為菩薩所現色相，化受用身非實報身。唯在彼天為顯此義，故言界也。」

Regarding "manifesting a tall and vast body in Akaniṣṭha": as a verse in the Ten-fascicle Laṅkāvatārasūtra states: "In the desire realm and formless realm, the Buddha does not attain Buddhahood there; in the highest heaven of the form realm, detached from desire, one attains bodhi." Now explaining the intention of this sūtra: "If we discuss the meaning of the true saṃbhogakāya, it is omnipresent throughout the dharmadhātu with no place where it is absent; yet saying that one attains Buddhahood with a body only in that heaven means it is a physical form manifested for bodhisattvas—a transformed saṃbhogakāya, not the true saṃbhogakāya. It is only in that heaven to illustrate this meaning; hence it speaks of the realm."