Manava Dharmashastra 3.117

3.117

na yathā daivāni karmāṇyevavārthānyevaṃ pitryaṃ nāma tatkarmatat— kiṃ tarhi—prathitā khyātā vedavidām pretakṛtyā pretopakriyā | vidhukṣaye | vidhuścandrastasya kṣaya amāvāsyā | 'tithikṣaya' iti pāṭhāntaram | 'vidhikṣaya' iti tu pāṭho nirduṣṭaḥ | evaṃ hi tatra yojanam—pitryaṃ nāma 'vidhi' coditaṃ karma—'kṣaye' gṛhe | tasmin karmaṇi pitrye | yuktasya tatparasya | nityaṃ karturupatiṣṭhate | pretakṛtyaiva | tasyāpi 'pretasya' 'kṛtyā'upakāraḥ śrāddhādiḥ putraiḥ kriyate | putrapautrādisantatyavicchedaḥ śrāddhaphalamanena prakāreṇa pratipādyate | na ca tatphalakāmasyāyamadhikāro, nityatvasya pratipāditatvāt | anye tvadhikārāntaramidaṃ santatyavicchedakāmasyecchanti | laukikīyaṃ kartavyatā, smārtetyarthaḥ |

This rite named “Pitrya,” performed on the Moonless Day is known as beneficial to the dead. To him who is intent on performing it, there always accrues benefit after death, offered according to human ordinances.—(127)

Medhātithi’s commentary (manubhāṣya):
‘Rites performed for the gods’ are not regarded as conferring any benefit on the gods; such, however, is not the case with this rite which is named ‘Pitrya.’ “What then?” It is ‘known,’ among people learned in the Vedas as ‘beneficial to the dead’—as conferring a benefit on the dead.

‘On the moonless night’—on the day on which there is no moon; i.e., the amārasyā day. Another residing is ‘vidhikṣaye.’ But the most faultless reading is ‘vidhuhṣaye.’ The meaning of the former would be that the rile named Pitryā is one that has been prescribed as to be done in the house.

‘Upon this’—upon this rite,—‘he who is intent’—he who is busy with its performance. To him there ‘always accrues,’ ‘benefit after death;’ i.e., for him also benefit after death is conferred by his sons performing the śrāddha and other rites for him.

What this means is that the continuity of the lines of one’s descendants—sons and grandsons—is the reward of performing śrāddhas. But this reward is not the incentive to its performance; as the rite has already been described as an obligatory one.

Others, however, have regarded this as indicating another incentive, for one desiring continuity of his line.

‘Offered according to human ordinances’:—i.e., this rite is performed according to rules laid down in the Smṛtis.—(127)