Shiva Purana

siva.sh/siva-purana/kotirudra-samhita/6/61

अद्यैव सप्तमी या सा गंगारूपास्ति तत्र वै।। इत्युक्त्वान्तर्दधे देवी सा गंगा मुनिसत्तमाः ।। ६१ ।।

sanskrit

Today is that seventh day. The river is in the form of Gaṅgā there.” Saying so the lady vanished. O excellent sages, she was Gaṅgā herself.

english translation

adyaiva saptamI yA sA gaMgArUpAsti tatra vai|| ityuktvAntardadhe devI sA gaMgA munisattamAH || 61 ||

hk transliteration by Sanscript