Chandogya Upanishad
Prapathaka II, Khanda 15
The mists gather, that is the hiṅkâra; the cloud has risen, that is the prastâva; it rains, that is the udgîtha; it flashes and thunders, that is the pratihâra; it stops, that is the nidhana. That is the Vairûpa Sâman, as interwoven in Parganya, the god of rain.