In the Daibontenno Monbutsu Ketsugikyo (Sutra on King Mahabrahma's Questions to the Buddha and Clarification of Doubts), it is written that there are Buddhist teachings known as Shobogenzo (literally, True Dharma-eye Treasury), Nehanmyoshin (Fine Mind of Nirvana) and Jissomuso (Undisclosed Subtle Dharma Gate), which were transmitted to Mahakasyapa outside the teachings without the use of written words.
大梵天王問仏決疑経の中に「正法眼蔵・涅槃妙心・実相無相の法門があり、文字を立てず教外に別伝して迦葉に付属する」とある。