That is based on the phrase, 'Jodo Chu Jobutsu shitsu kore hoshin' (浄土中成仏悉是報身, all the Buddha in Jodo are Hoshinbutsu), in "Daijo doshokyo" (The Mahayana Sutra on the Equal Nature).
それは『大乗同性経』の「浄土中成仏悉是報身」という文によっている。