Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.4.2

4.6.4.2

sa saṃhitaiḥ parvabhiḥ | idamannādyamabhyuttasthau yadidam prajāpaterannādyaṃ yadvai manuṣyāṇāmaśanaṃ taddevānāṃ vratam mahadvā idaṃ vratamabhūdyenāyaṃ samahāsteti tasmānmahāvratīyo nāma

With his joints thus restored, he approached this food, what food of Pragâpati there is,--for what eating is to men, that the vrata (fast-food, or religious observance generally) is to the gods. And because (they say), 'Great, indeed, is this vrata whereby he has raised himself,' therefore it is called Mahâvratîya.