Shiva Purana
आद्यम्वर्णमकारं च उकारमुत्तरे ततः ॥ मकारं मध्यतश्चैव नादांतं तस्य चोमिति ॥ १५ ॥
It consists of A, U and M in the middle, Bindu and Nāda at the end. That is Om.
english translation
AdyamvarNamakAraM ca ukAramuttare tataH ॥ makAraM madhyatazcaiva nAdAMtaM tasya comiti ॥ 15 ॥
hk transliteration by Sanscript