Brahma Sutras
ॐ श्रवणाध्ययनार्थप्रतिषेधात् स्मृतेश्च ॐ ॥ १-३-३८॥
And because the Smriti prohibits for the Sudra the hearing, study and acquisition of the meaning (of the Vedas).
english translation
OM zravaNAdhyayanArthapratiSedhAt smRtezca OM ॥ 1-3-38॥
hk transliteration by Sanscript