Skandapurana 16.206.71

16.206.71

atha śrāddhāvasāne tu āśiṣastatra dāpayet ..
dīrghā nāgāstathā nadyo viṣṇostrīṇi padāni ca ..
evameṣāṃ pramāṇena dīrghamāyuravāpnuyām .. 71 ..

Then, at the conclusion of the ancestral rite, he should have blessings pronounced there: / 'Long are the serpents, and likewise the rivers, and the three strides of Viṣṇu; / by the measure of these, may I attain a long life!' (71)