Charak Samhita
यदन्तरीक्षात्पततीन्द्रसृष्टं चोक्तैश्चपात्रैःपरिगृह्यतेऽम्भः। तदैन्द्रमित्येववदन्तिधीरा नरेन्द्रपेयंसलिलंप्रधानम् ॥२०१॥
Sages and wise people called rainwater as celestial water that is showered from the sky by Indra (the Vedic King of all Gods). This is the best kind of water and is ideal for consumption.
english translation
yadantarIkSAtpatatIndrasRSTaM coktaizcapAtraiHparigRhyate'mbhaH। tadaindramityevavadantidhIrA narendrapeyaMsalilaMpradhAnam ॥201॥
hk transliteration by Sanscript