Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.1.14

2.6.1.14

prokṣaṇīradhvaryurādatte | sa idhmamevāgre prokṣatyatha vedimathāsmai barhiḥ prayacanti tatpurastādgranthyāsādayati tatprokṣyopaninīya visraṃsya granthiṃ na prastaraṃ gṛhṇāti sakṛdu hyeva parāñcaḥ pitarastasmānna prastaraṃ gṛhṇāti

The Adhvaryu takes the lustral water, and sprinkles first the firewood, and then the altar. Thereupon they hand the sacrificial grass to him, and he puts it down (on the altar) with the knot to the east. Having thereupon sprinkled it and poured out (the lustral water on the lower ends of the grass-stalks), and untied the knot, he (at once) seizes the knot, not the prastara;--it is once for all that the fathers have departed: hence he does not take the prastara.