Chandogya Upanishad 4.8

'A diver-bird (Madgu, meant for Prâna) will declare to you another foot of Brahman.' (After these words of the Hamsa), Satyakâma, 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.

Then a diver flew near and said to him: 'Satyakâma.' He replied: 'Sir.'

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 Âyatanavat (having a home). 'He who knows this and meditates on the foot of Brahman, consisting of four quarters, by the name of Âyatanavat, 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 Âyatanavat.'