Mimamsasutra 107.2

107.2

atha kasmān na karmāṅgaṃ deśaḥ. yaḥ kṛṣṇamṛtikāprāyaḥ, sa āhnīnaibukādīnām. yathā - prācīnapravaṇe vaiśvadevena yajeteti.

Now, why is the place not an auxiliary element of the rite? That which is predominantly black soil belongs to Āhnīnaibuka and the like, as in: "One should sacrifice with the Vaiśvadeva on ground sloping to the east."