Garudapurana 1.213.144

1.213.144

tūṣṇīmeva tu śūdrasya sanamaskārakaṃ smṛtam /
adhyāpanaṃ brahmayajñaḥ pitṛyajñastu tarpaṇam //

for a Śūdra, it is traditionally said to be in silence, accompanied by an obeisance. / Teaching is the sacrifice to Brahman; the water-offering is the sacrifice to the ancestors;