Valmiki Ramayana Bala Kanda Sarga 43 Shloka 4

राम O Rama, तत: thereafter, हैमवती daughter of Himvat, सर्वलोकनमस्कृता adored by all worlds, ज्येष्ठा eldest (गङ्गा Ganga), तदा then, अति महत् mighty, रूपं form, दस्सहम् unbearable, वेगं speed, कृत्वा having assumed, आकाशात् from the sky, शिवे on the auspicious, शिवशिरसि on the head of Siva, अपतत् उत fell down.

Then Ganga, the eldest daughter of Himavat who is adored by all the worlds, assuming the form of a mighty river descended from the sky with unbearable speed on the auspicious head of Siva.