釋氏要覽 1.90

1.90

智度論云。涅槃有三門。一空門。二無相門三無作門何者空門。謂觀諸法無我我所。諸法從因緣生。無作者受者。是名空。今出家人。由此門入涅槃宅。故號空門子。

The Mahāprajñāpāramitopadeśa states: "Nirvāṇa has three gates: 1. the gate of emptiness; 2. the gate of signlessness; and 3. the gate of wishlessness. What is the gate of emptiness? It means contemplating all dharmas as without self or what belongs to self; all dharmas arise from causes and conditions, with no agent and no receiver—this is called emptiness. Now, renunciants enter the dwelling of nirvāṇa through this gate; hence they are called the sons of the gate of emptiness."