In Hinduism, Ekakshara, literally “the one syllable”, refers to the sacred monosyllable OM. Since the A is inherent in OM, meditation on the letter A became one of the most basic Buddhist contemplative practices, and it became particularly developed in the Japanese Shingon (Esoteric) School under the name of Ajikan.
The original Sanskrit source is in the concise Sutra of Transcendent Wisdom of One Single Letter (Prajnaparamita Ekakshari Sutra), here below:
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 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.
Close your eyes and say, “A”.
Click here to read an article on the history and development of Ajikan.
Follow this link for a documentary on the Pilgrimage to Koyasan, the main temple of the Shingon tradition and still a popular pilgrimage destination.