Chandogya Upanishad
Prapathaka IV, Khanda 6
'Agni will declare to you another foot of Brahman.' (After these words of the bull), Satyakâma, on the morrow, drove the cows (toward the house of the teacher). 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 to the east.
Agni said: 'Friend, I will declare unto you one foot of Brahman.' 'Declare it, Sir,' he replied. He said to him: 'The earth is one quarter, the sky is one quarter, the heaven is one quarter, the ocean is one quarter. This is a foot of Brahman, consisting of four quarters, and called Anantavat (endless).'