Manasara 6.54

6.54

catuṣkarṇaiśca karṇaiśca catuḥśaṅku samāyutam
evaṃ mahārthakaṃ kuryācchaṅkudāru pravakṣyate

and at the four corners, provided with four gnomons; / thus he should make it with great purpose; the wood for the gnomon will now be declared.