大般涅槃經疏 10.21

10.21

「迦葉白佛」去，第三科簡時。有問有答。問意云：佛滅後四十年隱沒於地，為當永沒？復更興耶？次佛答中云餘八十年者，一云：但正法餘八十年在時，前四十年興，後四十年已即滅。一解云：正法一千五百年，未滅八十，前四十興，後四十滅也。

From "Kāśyapa said to the Buddha" onward is the third [section], evaluating and discerning the time. There are questions and answers. The import of the question says: when it sinks into the earth for forty years after the Buddha's parinirvāṇa, does it sink permanently, or will it flourish once again? Next, in the Buddha's answer, where it says "the remaining eighty years": one says that only when eighty years of the true Dharma remain, it flourishes in the earlier forty years, and after the latter forty years it then perishes. Another explanation says: of the one thousand five hundred years of the true Dharma, when eighty years remain before its perishing, the earlier forty flourish and the latter forty perish.