"Bussetsu Muryoju-kyo Sutra" (Sutra of Immeasurable Life) teaches that Amitabha Nyorai received the teachings of 'Sejizaio-nyorai' when he was a Hozo bosatsu (Dharmakara Bodhisattva) and thought and promised the 48 vows for almost as long as eternity, later underwent more training to fulfill the promise and became a Buddha.
『仏説無量寿経』に、阿弥陀如来は法蔵菩薩時代に「世自在王如来」の教えを受け、ほとんど無限とも言える間、思惟して四十八願を誓い、その後、修行を積み誓願を完成させ仏となったと説かれる。