Valmiki Ramayana Bala Kanda Sarga 35 Shloka 18

हिमवान् Himavan, धर्मेण having followed his duty, त्रैलोक्यहितकाम्यया intent on doing welfare for three worlds, लोकपावनीम् sanctifying whole world, स्वच्छन्दपथगाम् flowing at her free will, तनयाम् daughter, गङ्गाम् Ganga, ददौ gave.

In order to do good to the three worlds Himavan, rooted in dharma gifted his eldest daughter Ganga who sanctifies the whole world by flowing at her free will৷৷