Bhagwad Gita, Upanishadas & Vedas mainly. Then there are other religious texts like Ramayana etc.
Ok.
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The concept of castes was first put forth in "Manu Smriti" which is not cosidered a religious/sacred text.
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We just gave references from the Gita about caste system. Or do you mean to say that social statuses of castes was set apart in Manu Smriti?