Śatapatha Brāhmaṇa ("of a hundred paths") 13.8.1.20

13.8.1.20

athoddhantavā āha sa yāvatyeva nivapsyantsyāttāvaduddhanyātpuruṣamātraṃ tvevoddhanyāttathāparasmā avakāśaṃ na karotyatho oṣadhiloko vai pitavra oṣadhīnāṃ ha mūlānyupasarpantyatho nedasyā antarhito'saditi

He then bids them cut out (the earth). He should cut it out to whatever extent he intends to raise (the sepulchral mound), but let him rather cut it out so as to be just of man's size: he thus leaves no room for another. For, on the one hand, the Fathers are the world of plants, and amongst the roots of plants they (are wont to) hide; and, on the other, (he does so) lest he (the deceased) should be separated from this (earth).