摩訶止觀 8.121

8.121

若遭疾病，因怖畏生死，知此病身酬於前業。若構生死，將來流轉復何窮極？苦集危脆，世世相隨，為之受惱；當求寂滅無相涅槃。是為因病起聲聞法界。

If, encountering illness, one is terrified of saṃsāra because of it, knowing that this diseased body is recompense for past karma; if one continues to construct saṃsāra, how could future transmigration ever end? Suffering and its origination are perilous and fragile, following one from lifetime to lifetime, causing one to suffer affliction; one should seek cessation and signless nirvāṇa. This is giving rise to the śrāvaka dharmadhātu due to illness.