Khandogya-Upanishad: Fourth Prapathaka, Eighth Khanda

(After these words of the Hamsa), Satyakima, on the morrow, drove the cows onward. And when they came towards the evening, he lighted a fire, penned the cows, laid wood on the fire, and sat down behind the fire, looking toward the east.

2. Then a diver flew near and said to him: “Satyakima.” He replied: “Sir.”

3. The diver said: “Friend, I will declare unto you one foot of Brahman.”

“Declare it, Sir,” he replied.

He said to him: “Breath is one quarter, the eye is one quarter, the ear is one quarter, the mind is one quarter. This is a foot of Brahman, consisting of four quarters, and called Ayatanavat (having a home).

4. “He who knows this and meditates on the foot of Brahman, consisting of four quarters, by the name of Ayatanavat, becomes possessed of a home in this world. He conquers the worlds which offer a home, whoever knows this and meditates on the foot of Brahman, consisting of four quarters, by the name of Ayatanavat.”