Revākhaṇḍa ascribed to the Vāyupurāṇa (RKV) 209.123

209.123

tata evaṃ vigāhyāpo mantrametamudīrayet /
tvaṃ narmade puṇyajale tavāmbhaḥ śaṅkarodbhavam //

Then, thus plunging into the waters, he should recite this mantra: / "You, O Narmadā of holy water, your water originates from Śaṅkara;