Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.1.17

1.2.1.17

athopalāmupadadhāti | dhiṣaṇāsi pārvateyī prati tvā parvatī vettviti kanīyasī hyeṣā duhiteva bhavati tasmādāha pārvateyīti prati tvā parvatī vettviti prati hi svaḥ saṃjānīte tatsaṃjñāmevaitaddṛṣadupalābhyāṃ vadati nedanyo 'nyaṃ hinasāta iti dyaurevaiṣā rūpeṇa hanū eva dṛṣadupalejihvaiva śamyā tasmācamyayā samāhanti jihvayā hi vadati

He then puts the upper mill-stone on (the lower one), with the text (Vâg. S. I, 19): 'A rock-born bowl art thou! May the rock acknowledge thee!' For this one being smaller is, as it were, the daughter (of the lower mill-stone); for this reason he calls it 'rock-born.' 'May the rock acknowledge thee!' he says, because one of the same kin acknowledges (receives the other): thereby he establishes an understanding between those two millstones, thinking 'they will not hurt one another!' This one, as it were, represents the sky; (or) the two mill-stones are, as it were, the two jaws, and the wedge is the tongue: that is why he beats (the mill-stones) with the wedge, for it is with the tongue that one speaks.