大乘起信論裂網疏 5.28

5.28

為除此執，明如來藏無有初際，無明依之生死無始。若言三界外更有眾生始起者，是外道經中說，非是佛教。以如來藏無有後際，證此永斷生死種子。

To eliminate this attachment, [the treatise] clarifies that the tathāgatagarbha has no initial limit; ignorance relies on it, so saṃsāra is beginningless. If one says that beyond the three realms there are sentient beings who have a first beginning, this is what is taught in non-Buddhist scriptures, not the Buddhist teaching. Because the tathāgatagarbha has no final limit, realizing this eternally severs the seeds of saṃsāra.