Skandapurana 6.3.209.123

6.3.209.123

tata evaṃ vigāhyāpo mantrametamudīrayet .
tvaṃ narmade puṇyajale tavāmbhaḥ śaṅkarodbhavam .. 209.123 ..

Then, having thus entered the waters, one should utter this mantra:
'O Narmadā of holy waters, your water arose from Śaṅkara.