Manava Dharmashastra 3.183

3.183

yata eteṣāṃ pitṝṇāmutpattirye ca pitaro yairupacaryāḥ | brāhmaṇena somapāḥ, kṣatriyeṇa haviṣmantaḥ ityādi | tatsarvamapyaśeṣata idānīmucyamānaṃ nibodhata budhyadhvam | niyamairityanuvādaḥ | pūrvameva vihitatvāt “niyatātmā bhavet ” iti | bahuvacanaṃ bahutvānniyamānām |

Now fully learn from whom all these have their origin and who are to be worshipped, by whom and by what roles.—(193)

Medhātithi’s commentary (manubhāṣya):
He from whom these—Pitṛs—‘have their origin,’ and ‘who’—which Pitṛs—‘have to be worshipped by whom,’ i.e., the ‘Somapa’ Manes by Brāhmaṇas, the ‘Haviṣmat’ Pitṛs by Kṣatriyas, and so forth;—all this ‘learn fully,’ as described now.

‘Rules;’—this is a mere reiterative reference; the rules having been already laid down in the words, ‘one should remain self-controlled, &c., &c.’ The plural number is due to there being a large number of rules.—(193)