In which sutra does the Buddha just say, "A"?

I'm reading a book called "The Heart Attack Sutra" and the author mentions this short sutra, which must surely be the most hilarious and profound of all Buddhist teachings. Unfortunately the author does not give much more info about it before moving on. Any ideas where I might find it?

I noticed it's referenced frequently by Arthur Fonzarelli (AKA The Fonz) in the show "Happy Days" but that's all I could find.

Edit: finally found something. https://www.himalayanart.org/search/set.cfm?setID=4483

The [Sutra of the] Blessed Perfection of Wisdom, The Mother of All the Tathagatas, In One Letter

Excerpt below:

"Ananda, do receive, for the sake of the weal and happiness of all beings, this perfection of wisdom in one letter, A."

Submitted November 22, 2019 at 04:29AM by mean_ruse https://ift.tt/37lSV4R