Chandogya Upanishad
Prapathaka II, Khanda 4
Let a man meditate on the fivefold Sâman in all waters. When the clouds gather, that is the hiṅkâra; when it rains, that is the prastâva that which flows in the east , that is the udgîtha that which flows in the west , that is the pratihâra the sea is the nidhana.