Śatapatha Brāhmaṇa ("of a hundred paths") 11.4.1.13

11.4.1.13

atha yadanuvākyāmanūcya yājyayā yajati tasmādadhara evāgre jāyante'thottare'tha yadgāyatrīmanūcya triṣṭubhā yajati tasmādaṇīyāṃsa evādhare prathīyāṃsa uttare'tha yatprāñcāvāghārāvāghārayati tasmāddaṃṣṭrā varṣīyāṃso'tha yatsacandasāveva saṃyājye bhavatastasmātsamā eva jambhyāḥ

And inasmuch as, after uttering the invitatory formula, he offers with the offering-formula, therefore the lower (teeth) grow first, then the upper ones; and inasmuch as, after uttering a gâyatrî verse as invitatory formula, he offers with a trishtubh verse, therefore the lower (teeth) are smaller, and the upper ones broader; and inasmuch as he pours out the two libations of ghee in a forward direction, therefore the incisors are larger; and inasmuch as the two samyâgyâs are in the same metre, therefore the molars are of equal size.