Vaishnava Dharmashastra 21.4

21.4

bhuktavatsu brāhmaṇeṣu dakṣiṇayābhipūjiteṣu pretanāmagotrābhyāṃ dattākṣayyodakaḥ caturaṅgulapṛthvīḥ tāvadantarāḥ tāvadadhaḥkhātāḥ vitastyāyatāḥ tisraḥ karṣūḥ kuryāt ||

The Brâhmanas having taken food and having been honoured with a gift, let him offer, as imperishable food, water to the Brâhmanas, after having called out the name and Gotra of the deceased; and let him dig three trenches, each four Angulas in breadth, their distance from one another and their depth also measuring (four Angulas), and their length amounting to one Vitasti (or twelve Angulas).