Śaṅkaraon Brahma Sūtra 4.3.3

The AV/Andogya continues, ‘From Aditya to the moon, from the moon to lightning.’ Here Varu//a (mentioned in the Kaushitaki-upan.) has to be brought in so that above that lightning he goes to the world of Varu;/a. For there is a connexion between lightning and Varuwa; the broad lightnings dance forth from the womb of the clouds with the sound of deep thunder, and then water falls down. And a Brahma;/a also says, ‘It lightens, it thunders, it will rain’ (A"//. Up. VII, 11, 1). But the lord of all water is Varu;m, as known from 5ruti and Srnr/ti. And above Varu;/a there come Indra and Pra^apati, as there is no other place for them, and according to the force of the text, as it stands, Varu;/a and so on should be inserted at the end, for that reason also that they are merely additional, no particular place being assigned to them. And lightning is the end of the road beginning with light. So that Varuwa and so on are to be placed after lightning.