Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.1.10

6.2.1.10

tadyadiṣṭvā paśunāpaśyat | tasmādiṣṭakāstasmādiṣṭvaiva paśuneṣṭakāḥ kuryādaniṣṭakā ha tā bhavanti yāḥ purā paśoḥ kurvantyatho ha tadanyadeva

And inasmuch as he saw them after offering (ishtvâ) the animal, therefore they are bricks (ishtakâ). Hence one must make the bricks only after performing an animal sacrifice; for those which are made before (or, without) an animal sacrifice are 'anishtakâ.' And, moreover, there is this other (consideration).