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?