The shortest of all the Upanishads is known as the Mandukya Upanishad. It's only twelve verses long, and it isn't mentioned much in Hindu scripture, but it grew into prominence after Gaudapada, the early Advaita Acharya who was Adi Shankaracharya's guru's guru, composed a famous Karika or commentary on it.
Now Madhvacharya, exponent of the Dvaita school, believed that at least the first chapter of what is considered as Gaudapada's Karika wasn't composed by Gaudapada at all. Instead, he thought that it was composed by Brahma, who was commenting on the Mandukya Upanishad as it was originally being recited by Varuna the ocean god; here is what Madhvacharya says in his commentary on the Mandukya Upanishad:
In the form of a Manduka (a frog), Varuna eulogized the four-fold Sriman Narayana, meditating on Sriman Narayana, encompassed by Aum, Varuna, praised immutable Sri Hari, thus has it been said in Padma Purana.... Thus did Varuna himself, in the form of Manduka (frog) realize himself. Thus has it been mentioned in Harivamsha.... The man~ which were „seen’ by Brahma are the premises, O Lord of the Waters (Varuna). „A~Oto Slaaok:‟ thus is spoken piece by piece. Thus has it been said in Garuda Purana.
So my question is, what is the basis of Madhvacharya's claim that the seer of the Mandukya Upanishad is Varuna the ocean god in the form of a frog (Manduka), and that the commentator is Brahma? He cites quotes from the Padma Purana, Harivamsa, and Garuda Purana, but does anyone know where exactly they are in those texts? The quotes are written out in Devanagari script in pages 1 and 2 here if that helps.
By the way, on a side note I'm a Sri Vaishnava, and it seems that at least some Sri Vaishnava Acharyas have similar beliefs about the beginning of Gaudapada's Karika.
