Chandogya Upanishad
'A Hamsa (flamingo, meant for the sun) will declare to you another foot of Brahman.' (After these words of Agni), 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 Hamsa flew near and said to him: 'Satyakima.' He replied: 'Sir.'
The Hamsa said: 'Friend, I will declare unto you one foot of Brahman.' 'Declare it, Sir,' he replied. He said to him: 'Fire is one quarter, the sun is one quarter, the moon is one quarter, lightning is one quarter. This is a foot of Brahman, consisting of four quarters, and called Gyotishmat (full of light).
'He who knows this and meditates on the foot of Brahman, consisting of four quarters, by the name of Gyotishmat, becomes full of light in this world. He conquers the worlds which are full of light, whoever knows this and meditates on the foot of Brahman, consisting of four quarters, by the name of Gyotishmat.