妙法蓮華經玄義 7.67

7.67

八、本涅槃者，本時所證斷德涅槃，亦是本時應處同居、方便二土，有緣既度，唱言入滅，即本涅槃也。

Eighth, the original nirvāṇa: the nirvāṇa of the virtue of severance realized at the original time; it is also that at the original time, responding in the two lands of common residence and expedients, when those with conditions had been delivered, he proclaimed entering extinction. This is the original nirvāṇa.