Mandukya Upanishad:

The Mandukya Upanishad directs the seeker to adopt a fixed method of worship. Most meditators have a picture of ૐ in their sadhana room. But a clear understanding of what is the relationship between Niranjan, the formless Brahman, the embodied Brahman and Om and why Brahman can be attained by worshiping 'Aum' is essential.

By explaining its relationship with the four padas, four charanas or four divisions of Parabrahman and the four quantities of Om, the sage in this Upanishad has explained in a very simple way the esoteric knowledge of how a seeker can ascend through the spiritual path by doing ritualistic practice with different names of Brahma and different quantities of Om.