अकारो मुखः सर्वधर्माणामाद्यनुत्पन्नत्वात्
Sanskrit translates as: "The letter 'a' is an opening because of the primal quality of non-arising of all mental phenomena." [my translation]
First line of the Arapacana acrostic as found in the Pañcaviṃśatisāhasrikā Prajñāpāramita Sūtra (The Perfection of Wisdom Sūtra in 25,000 lines). More info on the Visible Mantra Arapacana page.
Each line of this acrostic has a key word which begins with the the corresponding letter of the Gāndhārī alphabet (as envisaged by Sanskrit speakers). The whole collection make up a meditation practice in which the practitioner tries to see the true nature of experience as empty of independent existence - i.e. that all experiences are conditioned by the meeting of mind and mental objects.
(Thanks again to Dksht for spotting errors. I think both the Siddhaṃ and Roman are correct now)