One verse

Srimad Bhagavatam 11.17.19

11.17.19

शुश्रूषणं द्विजगवां देवानां चाप्यमायया । तत्र लब्धेन सन्तोष: शूद्रप्रकृतयस्त्विमा: ॥ १९ ॥

śuśrūṣaṇaṁ dvija-gavāṁ devānāṁ cāpy amāyayā tatra labdhena santoṣaḥ śūdra-prakṛtayas tv imāḥ

śuśrūṣaṇam
service
dvija
of the brāhmaṇas
gavām
of the cows
devānām
of worshipable personalities such as the demigods and the spiritual master
ca
also
api
indeed
amāyayā
without duplicity
tatra
in such service
labdhena
with that which is obtained
santoṣaḥ
complete satisfaction
śūdra
of the śūdras
prakṛtayaḥ
the natural qualities
tu
indeed
imāḥ
these

Service without duplicity to the brāhmaṇas, cows, demigods and other worshipable personalities, and complete satisfaction with whatever income is obtained in such service, are the natural qualities of śūdras.