Hello LuckyUK, hi Marrish,
Your quote does appear in the Vedas, it is found in Ṛg Veda 1.164.46. The full verse as it appears in the Ṛg Veda is given below (complete with the Vedic accent):
इन्द्रं॑ मि॒त्रं वरु॑णम॒ग्निमा॑हु॒रथो॑ दि॒व्यः स सु॑प॒र्णो ग॒रुत्मा॑न् ।
एकं॒ सद्विप्रा॑ बहु॒धा व॑दन्त्य॒ग्निं य॒मं मा॑त॒रिश्वा॑नमाहुः ॥४६॥
The rest of Ṛg Veda 1 is available online, in fact sanskritweb has a lovely .pdf copy of the Ṛg Veda in a very professional font, if you want to see what that verse would more likely have looked like when written down. Anyway, the transliteration is (according to the Western system):
46. Índraṃ mitráṃ váruṇam agním āhur átho divyáḥ sá suparṇó garútmān;
Ékaṃ sád víprā bahudhā́ vadanty agníṃ yamáṃ mātaríśvānam āhuḥ.
The translation of the verse that I would give is:
46. They called him Indra, Mitra, Varuṇa, Agni; yea, he is heavenly Garuḍa, who has beautiful wings.
That which is One, the sages speak of as Multifarious; they called him Agni, Yama, Mātariśvan.
Wikisource has a complete translation of Ṛg Veda 1.164 if you're interested.
Given the context of the rest of the verse I think it's pretty clear that ékaṃ (One) refers to god (Indra, Mitra, Varuṇa, Agni, Garuḍa, Yama and Mātariśvan are all Vedic gods) and this verse is more about god, than about Truth, but I am not a scholar of the Vedas, so perhaps it's open to interpretation. I am aware from what I came across when looking up this verse that it comes up very often in debates about monotheism (and, I think a little more accurately, monolatrism) in Hinduism and it is more generally, I think, thought to be about god.
Apart from that I just want to second what Marrish has said, his corrections, explanations and translations are - as far as I can see - perfect.
To add to what Marrish has said, though, there are two things that we needed to correct in the original. The first of these is saṃdhi, which Marrish has already spoken about. As he says it is really just phonology, very many languages have Saṃdhi or something similar to it. In Sanskrit the rules get very complicated, but - at heart - they are merely rules of pronunciation. When you speak English, you know that we can express the plural with a [s] sound (as in cake-s) a [z] sound (as in road-s) or the sound [ɨz] (as in rose-s). The reason why we have [s] in some sounds and [z] in others is that the rules of English pronunciation tell us that we can't have the voiced sound [z] ('voiced' means that our vocal chords vibrate when we pronounce the sound) coming after a voiceless sound such as [k] or [p] (e.g. lap-s). There is a similar rule in Sanskrit which says that you can't have the voiceless sound t coming before the voiced sound v (as in sát víprā, which is what you would have without saṃdhi) instead we get the voiced sound d.
The second is purely a matter of orthography, when writing Sanskrit in Devanagari, the convention says that any words that can be written together must be written together. In other words, you only use a space when you have to. Unlike English, where all words are separated by spaces, it is possible to have an entire line of a śloka written all together. This is why we have सद्विप्रा॑ not सद् विप्रा॑. You may have noticed that western transliteration conventions do not observe this convention, instead choosing to separate the words, as I have done above.