Narasimhapurana 58.10

58.10

etad āsevamānas tu sa svargī syān na saṃśayaḥ
varṇatrayasya śuśrūṣāṃ kuryāc chūdraḥ prayatnataḥ //

practicing this, he will go to heaven without doubt. / A śūdra should diligently perform service to the three [higher] castes;