This chapter of the Shatapatha Brahmana of the Yajur Veda describes how Brahma created the Devas and Asuras from his breath. After he created both, he realized that the Asuras were evil, so he decided to fight them. The Shatapatha Brahmana says that while it may seem like the Devas are defeating the Asuras, really Brahma is the one responsible for defeating the Asuras, and he's just using the Devas to accomplish this:
Desirous of offspring, he went on singing praises and toiling. He laid the power of reproduction into his own self. By (the breath of) his mouth he created the gods: the gods were created on entering the sky; and this is the godhead of the gods (deva) that they were created on entering the sky (div). Having created them, there was, as it were, daylight for him; and this also is the godhead of the gods that, after creating them, there was, as it were, daylight (diva) for him.
And by the downward breathing he created the Asuras: they were created on entering this earth. Having created them there was, as it were, darkness for him. He knew, 'Verily, I have created evil for myself since, after creating, there has come to be, as it were, darkness for me.' Even then he smote them with evil, and owing to this it was that they were overcome; whence people say, 'Not true is that regarding (the fight between) the gods and Asuras which is related partly in the tale and partly in the legend; for it was even then that Pragâpati smote them with evil, and it was owing to this that they were overcome.'
Therefore it is with reference to this that the Rishi has said, 'Not for a single day hast thou fought, nor hast thou any enemy, O Maghavan illusion is what they say concerning thy battles; no foe hast thou fought either to-day or aforetime.'
But my question is about the quote given at the end of this passage. In the Brahmanas, Aranyakas, and Upanishads, whenever the text says "a Rishi has said this", what it means is that it's quoting a Vedic verse heard during meditation by a Rishi, i.e. a mantra occurring in the Samhitas of the Vedas. So my question is, where is the Vedic verse that says that Indra has not fought (of his own accord) for a single day?
If it helps, here's the verse in Sanskrit.
na tvaṃ yuyutse katamaccanāharna te'mitro maghavankaścanāsti |
māyetsā te yāni yuddhānyāhurnādya śatruṃ na nu purā yuyutsa ||
I at least found the second half of this verse in Book 10 Hymn 54 of the Rig Veda:
yadacarastanvā vāvṛdhāno balānīndra prabruvāṇo janeṣu |
māyet sā te yāni yuddhānyāhurnādya śatruṃ nanupurā vivitse ||
When thou wast roaming, waxen strong in body, telling thy might, Indra, among the people,
All that men called thy battles was illusion: no foe hast thou to-day, nor erst hast found one.
As you can see in the Rig Veda Anukramani given in my answer here, this is a hymn heard during Tapasya by the sage Brihaduktha Vamadevya. So he may be the Rishi alluded to in the Shatapatha Brahmana. But where in the Vedic Samhitas is the first half of the verse is from?
By the way, on a side note it's not clear whether the four-headed god Brahma is being described here, because there's an ambiguity between Prajapati and Parameshthin Prajapatya which I discuss in my question here.