Idol Worship Prohibited in Vedas
1. An Introduction
The following are some references from Hindu scriptures. The following are only few references, you may find many more once you go through it.
Most of the Hindus denies some Purana's (myth) and Ithihasa's(like Ramayana, Mahabharatha) which contains more imaginations. Besides Purana and Ithihasa, the main Hindu scriptures are Bhagawat Gita, Vedas, Upanishads and Manu Xmriti (Manu Smirthi).
Bhagavad Gita : Most popular and the maximun number of translations available amongst all the religious scriptures of the Hindus.
Vedas : Amongst all the religious scriptures of the Hindus, the most sacred are the Vedas. Principally there are four Vedas and amongst all the Vedas, the Rigveda is the most sacred and oldest.
Manu Smirthi : The Holy Book and Hindu Book of Laws
2. From Vedas
Prohibition of Idol Worship in Yajurveda
"There is no image of him. He is unborned and He should be worshipped" (Yajurveda, Chapter 32, Verse 3)
"god is bodyless and pure" (Yajurveda, Chapter 40, Verse 8)
"They are entering darkness, those who worship the natural things(like air, water, fire etc.), they are sinking more in darkness who worship created things." (Yajurveda, Chapter 40, Verse 9)
Monotheism in Rigveda
"Say this, all one God in many names" (Rigveda, Book 1, Hymn 164, Verse 46)
"Do not worship anyone besides him alone, praise him alone" (Rigveda, Book 8, Hymn 1, Verse 1)
"Praise him would matchless and alone" (Rigveda, Book 6, Hymn 45, Verse 16)
3. From Bhagavad Gita
"Those whose intelligence has been stolen by material desires, they worship demigods" (Bhagavad Gita, Chapter 7, Verse 20)
4. From Upanishad
Prohibition of Idol Worship in Upanishad
"There is no likeness of him" (Svetasvatara Upanishad, Chapter 4, Verse 19)
"His form cannot be seen, No one can see him with the eyes" (Svetasvatara Upanishad, Chapter 4, Verse 20)
Monotheism in Upanishad
"God is one and only, not a second" (Chandogya Upanishad, Chapter 6, Section 2, Verse 1)
"Of him (God), there is no parents, no Lord" (Svetasvatara Upanishad, Chapter 6, Verse 9)
Monotheism in Vedantha
"There is only one god, Not a second one, not at all, not at all, not in a least bit" (BrahmaSutra)
** Rigveda - English Translation by Satyaprakash Narayan and Satyakam Vidhyarangan
** Yajurveda - English Translation by Devichand
** Upanishad - English Translation by S. Radhakrishnan