Where does the Rig Veda describe realizing supreme truth through Vishnu's name?

Question

In N Raghunathan's forward to a translation of the Vishnu Sahasranamam, he gives a purported quote from the Rig Veda:

The Rigveda ... affirms the potency of the Divine-Name in such verses as the following of Dirghatamas:

Asya jananto Nama-child-viviktana

mahaste Visnoh sumatim bhajamahe

Which means, "O ye who wish to gain realization of the supreme truth, utter the name of Visnu at least once in the steadfast faith that it will lead you to such realization".

My question is, where in the Rig Veda is this verse from? It says it is a verse of Dirghatamas. Well, as you can see in the Anukramani index for Rig Veda Book 1 in my answer here, the sage Dighatamas Aucatya is the seer of hymns 140-164 of Rig Veda Book 1. You can read Rig Veda Book 1 here; I couldn't find the verse, at least at first glance.

Wikipedia also quotes this verse:

From the oldest scriptural text in Hinduism, the Rig Veda; V.I.15b.3, it states: "O ye who wish to gain realization of the Supreme Truth, utter the name of "Vishnu" at least once in the steadfast faith that it will lead you to such realization."

But I'm not sure what "V.I.15b.3" means. The Rig Veda is divided into 10 books (Mandalas), each book is divided into a bunch of hymns (suktas), and each hymn is composed of a bunch of verses (richas). So why is it giving four numbers?

Answer 1

I checked telugu version of Vedas I have with me.

Mandala 1, hymn 156, Rishi - Dirghatama, devata - vishnu

Above meaning is not there anywhere in the 5 hymns of this verse.

What I know from the books I have studied on Hindu philosophy is that - Vishnu is one of 33 devatas in Vedas. There are hundreds of hymns for Indra, Agni, Varuna but for Vishnu and Rudra (shiva) there are very few. So they are the least important devatas in Vedas. Later during puranic period (mythology), the character of Vishnu was raised above rest of the devatas with stories such as Indra losing war with asuras and going to Vinshnu for saran (help) etc.,

Answer 2

I'm still not sure what Wikipedia meant by "V.I.15b.3", but in any case I found the line in hymn 156 of Mandala 1 of The Rig Veda:

1. FAR-SHINING, widely famed, going thy wonted way, fed with the oil, be helpful. Mitra-like, to us. So, Viṣṇu, e’en the wise must swell thy song of praise, and he who hath oblations pay thee solemn rites.
2. He who brings gifts to him the Ancient and the Last, to Viṣṇu who ordains, together with his Spouse, Who tells the lofty birth of him the Lofty One, shall verily surpass in glory e’en his peer.
3. Him have ye satisfied, singers, as well as ye know, primeval germ of Order even from his birth. Ye, knowing e’en his name, have told it forth: may we, Viṣṇu, enjoy the grace of thee the Mighty One.
4. The Sovran Varuṇa and both the Aśvins wait on this the will of him who guides the Marut host. Viṣṇu hath power supreme and might that finds the day, and with his Friend unbars the stable of the kine.
5. Even he the Heavenly One who came for fellowship, Viṣṇu to Indra, godly to the godlier, Who Maker, throned in three worlds, helps the Āryan man, and gives the worshipper his share of Holy Law.

The Sanskrit version is transliterated slightly differently than how Raghunathan gives it: "āsya jānanto nāma cid vivaktana mahaste viṣṇo sumatiṃ bhajāmahe". And as you can see above, Griffiths translates it as "Ye, knowing e’en his name, have told it forth: may we, Viṣṇu, enjoy the grace of thee the Mighty One."

And just as N. Raghunathan says, the sage who heard this hymn from the gods was Dirghatamas Aucatya; see hymn 156 in the Anukramani for Rig Veda Book 1 in my answer here.