Valmiki Ramayana 1.41.19
गंगा हिमवतो ज्येष्ठा दुहिता पुरुषर्षभ ।तस्याम् कुरु महाबाहो पितॄणाम् तु जल क्रियाम्॥
gaṃgā himavato jyeṣṭhā duhitā puruṣarṣabha |tasyām kuru mahābāho pitṝṇām tu jala kriyām ||
Our plain-language meaning of this verse is waiting for scholar review.
Open in the book