Valmiki Ramayana Bala Kanda Sarga 43 Shloka 38
Original Shlokaततस्तुष्टो महातेजाश्श्रोत्राभ्यामसृजत् पुन:।।1.43.38।। तस्माज्जह्नुसुता गङ्गा प्रोच्यते जाह्नवीतिच।
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तत: thence, तुष्ट: highly pleased, महातेजा: highly lustrous Jahnu, पुन: again, श्रोत्राभ्याम् from both his ears, असृजत् released, तस्मात् therefore, गङ्गा Ganga, जह्नुसुता daughter of Jahnu, जाह्नवीति Jahnavi, प्रोच्यते is being called.
The brilliant (Jahnu) was pleased. He released her through both his both ears. Hence Ganga is called Jahnavi, daughter of Jahnu.