These were described in 'Shojubun' of the "Muryoju-kyo Bussetsu Muryoju-kyo Sutra" (Sutra of Infinite Life, translated by Kosogai), which is the Konpon kyoten (primal scripture) of the Jodo (Pure Land) Sect.
浄土教の根本経典である『無量寿経仏説無量寿経』（康僧鎧訳）「正宗分」に説かれている。