Chandogya Upanishad
Prapathaka II, Khanda 2
Let a man meditate on the fivefold Sâman as the five worlds. The hiṅkâra is, the earth, the prastâva the fire, the udgîtha the sky, the pratihâra the sun, the nidhana heaven; so in an ascending line.
In a descending line, the hiṅkâra is heaven, the prastâva the sun, the udgîtha the sky, the pratihâra the fire, the nidhana the earth.