Patanjali Yog Sutras

patanjali yoga sutra
samadhipad
Sutra 27

sutra 27 tasya vachakah pranavah
tasya vachakah pranavah

Tasya: His (of that ishvar)
Vachakah: Description, name, calling
Pranavah: Om

That Ishvar's name (dhwani) is om.

Om is the essence of Creation, the primordial sound, which is constant and has no beginning or end, from which everything emerges and to which everything returns. That Om is the frequency which corresponds to ishvar, the route to ishvar.