Ugra Kangyur, p. 294
The "One Letter" Prajna Paramita

In Sanskrit: Eka-Aksara Maha-Namah Sarva-Tathagata Prajna-Paramita The Great "One Letter" Perfection of Transcendent Awareness, Mother, of All the Tathagatas Homage in full prostration to the great Mother Perfection of Transcendent Awareness. Thus was heard by me: at one time the Supramundane Victor resided on Vulture Heap at Raja-griha together with twelve hundred and fifty great monastic assemblies and many hundred thousand millions of ten millions Bodhisattvas. At that time the Supramundane Victor taught Ayusmat Ananda: §´You should memorize the "One Letter" version of Lady Perfection of transcendent Awareness, as a cause of benefit and happiness of sentient beings. It is like this: "A". With these words did the Supramundane Victor instruct. Ayusmat Ananda and all these monastics, Bodhisattvas and Mahasttvas gained insight into the Perfection of Transcendant Awareness and rejoiced. They exhuberantly praised what the Supramundane Victor had taught. The Perfection of Transcendent Awareness, Mother of all the Tathagatas, in its "One Letter" Version is hereby completed.