Thereafter, service was manifested from the legs of the Personality of Godhead for the sake of perfecting the religious function. Situated on the legs are the śūdras, who satisfy the Lord by service.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पद्भ्यां भगवतो जज्ञे शुश्रूषा धर्मसिद्धये । तस्यां जातः पुरा शूद्रो यद्वृत्त्या तुष्यते हरिः
padbhyāṃ bhagavato jajñe śuśrūṣā dharmasiddhaye | tasyāṃ jātaḥ purā śūdro yadvṛttyā tuṣyate hariḥ
SB 3.6.33