Mandukya Upanishad, second verse:

Text

सर्वं ह्येतद् ब्रह्मायमात्मा ब्रह्म सोऽयमात्मा चतुष्पात् ॥ २ ॥

sarvaṃ hyetad brahmāyamātmā brahma so 'yamātmā catuṣpāt || 2 ||

2. All this is verily Brahman. This Ātman is Brahman. This Ātman has four quarters.

Shankara Bhashya (commentary)

All this is verily Brahman. All that has been said to consist merely of Aum (in the previous text) is Brahman. That Brahman which has been described1 (as existing) inferentially2 is now pointed out, as being directly3 known, by the passage, “This Self is Brahman”. The word this, meaning that which appears divided into four quarters,4 is pointed out as the innermost Self, with a gesture5 (of hand) by the passage, “This is Ātman”. That Ātman indicated by Aum, signifying both the higher and the lower Brahman, has6 four quarters (Pādas), not indeed, like the four feet (Pādas) of a cow,7 but like the four quarters (Pādas) of a coin8 known as Kārṣāpaṇa. The knowledge of the fourth (Turīya) is attained by merging the (previous) three, such as Viśva, etc., in it in9 the order of the previous one, in the succeeding one. Here10 the word ‘Pāda’ or ‘foot’ is used in11 the sense of instrument. The word ‘Pāda’ is again used in the sense of an object when the object to be achieved is the fourth (Turīya).