Chandogya Upanishad
Prapathaka II, Khanda 3
Let a man meditate on the fivefold Sâman as rain. The hiṅkâra is wind (that brings the rain); the prastâva is, 'the cloud is come;' the udgîtha is, 'it rains;' the pratihâra, 'it flashes, it thunders;'