Manusmriti
विप्रसेवैव शूद्रस्य विशिष्टं कर्म कीर्त्यते । यदतोऽन्यद् हि कुरुते तद् भवत्यस्य निष्फलम् ॥ १२३ ॥
The service of Brahmanas alone is declared (to be) an excellent occupation for a Sudra; for whatever else besides this he may perform will bear him no fruit.
english translation
viprasevaiva zUdrasya viziSTaM karma kIrtyate । yadato'nyad hi kurute tad bhavatyasya niSphalam ॥ 123 ॥
hk transliteration by Sanscript