Matsyapurana 81.14

81.14

caturasraṃ samantācca ratnimātramudak‌plavam.
ślakṣṇaṃ hṛdyaṃ ca parito vipratrayasamāvṛtam.. 81.13 ..

The altar should be a square in shape, and of the dimensions of a cubit, and should slant towards the north. It must be smooth and soft, surrounded by three small mud-walls.—13.