I think this probably is the shortest sutra ever:
Single Letter Transcendent Wisdom Sutra:
Prajnaparamita Ekashari Sutra
Homage to Prajnaparamita!
Thus have I heard: At one time the Lord dwelt at Rajagrha, on the Vulture Peak, together with a large congregation of renunciates, with 1,250 renunciates, and with countless bodhisattvas. At that time the Lord addressed Venerable Ananda:
"Ananda, do receive, for the sake of the welfare and happiness of all sentient existence, this perfection of wisdom in one letter, the letter 'a'."
Thus spoke the Lord. The Venerable Ananda, the large congregation of renunciates, the assembly of bodhisattvas, and the whole world with its devas, humans, asuras, and heavenly musicians, rejoiced at the teaching of the Lord.
Unless you shave off the intro and end, which are quite superfluous, and are left standing with the "A". The brevity of that one would be hard to beat!
Kåre A. Lie