Chandogya Upanishad 2.15

abhrāṇi saṃplavante sa hiṅkāraḥ megho jāyate sa prastāvaḥ varṣati sa udgīthaḥ vidyotate stanayati sa pratihāraḥ udgṛhṇāti tan nidhanam etad vairūpaṃ parjanye protam

The mists gather, that is the hinkâra; 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.

sa ya evam etad vairūpaṃ parjanye protaṃ veda virūpāṃś ca surūpāṃś ca paśūn avarundhe sarvam āyur eti jyog jīvati mahān prajayā paśubhir bhavati mahān kīrtyā varṣantaṃ na nindet tad vratam

He who thus knows the Vairûpa as interwoven in Parganya, obtains all kinds of cattle (virûpa), he reaches the full life, he lives long, becomes great with children and cattle, great by fame. His rule is, 'Never complain of the rain.'