Manava Dharmashastra 1.91

1.91

prabhuḥ prajāpatirekaṃ karma śūdrasyādiṣṭavān | eteṣāṃ brāhmaṇakṣatriyavaiśyānāṃ śuśrūṣā tvayā kartavyā| anasūyayā'nindayā | cittenāpi tadupari viṣādo na kartavyaḥ| śuśrūṣā paricaryā tadupayogi karmakaraṇaṃ śarīrasaṃvāhanādi taccittānupālanam |
etaddṛṣṭārthaṃ śūdrasya | avidhāyakatvāccaikameveti na dānādayo niṣidhyante | vidhireṣāṃ karmaṇāmuttaratra bhaviṣyati | ataḥ svarūpaṃ vibhāgena yāgādīnāṃ tatraiva darśayiṣyāmaḥ |

For the Śūdra the Lord ordained only one function: the ungrudging service of the said castes. (91).

Medhātithi’s commentary (manubhāṣya):
‘The Lord’, Prajāpati, ordained ‘one function, for the Śūdra’; [in the form]—‘Thou shalt perform the ‘service of the said castes,’—i.e., of the Brāhmaṇa, Kṣatriya and Vaiśya;—‘Ungrudging’—i.e., without complaining; no resentment should be felt even in the mind.

‘Service’ stands for attending; i.e., doing of acts conducive to their convenience; such as massage of the body, and the obeying of wishes.

What is mentioned here is only such function of the Śūdra as leads to visible results (in the shape of livelihood); and in as much as the phrase ‘one only’ is not injunctive, it does not preclude the giving of gifts (and such other acts, leading to invisible results); specially as we shall find later on the actual injunction of these acts (for the Śūdra also). And it will be on that occasion that we shall set forth the classification of sacrifices and other acts (as to which of these should be done by which castes, and so forth). (91).