Shiva Purana
त्रैवर्णिकानामेवात्र श्रुत्याचारसमन्वयः ॥ शुश्रूषामात्रसारा हि शूद्राः श्रुतिबहिष्कृताः ॥ २२ ॥
The Vedas enjoin rituals for the first three castes. The Śūdras are excluded since their only activity is service.
english translation
traivarNikAnAmevAtra zrutyAcArasamanvayaH ॥ zuzrUSAmAtrasArA hi zUdrAH zrutibahiSkRtAH ॥ 22 ॥
hk transliteration by Sanscript