Shiva Purana
ब्रह्मक्षत्रियवैश्यानां शुश्रूषुः शूद्र उच्यते । कर्षको वृषलो ज्ञेय इतरे चैव दस्यवः ६ ।
A person rendering service to Brahmins, Kṣatriyas and Vaiśyas is called a Śūdra. A working agriculturist is a Vṛṣala and the others are Dasyus.
english translation
brahmakSatriyavaizyAnAM zuzrUSuH zUdra ucyate । karSako vRSalo jJeya itare caiva dasyavaH 6 ।
hk transliteration by Sanscript