Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.1.13

3.3.1.13

sā yā babhruḥ piṅgākṣī | sā somakrayaṇī yatra vā indrāviṣṇū tredhā sahasraṃ vyairayetāṃ tadekātyaricyata tāṃ tredhā prājanayatāṃ tasmādyo 'pyetarhi tredhā sahasraṃ vyākuryādekaivātiricyeta

One that is brown, with red-brown eyes, is (fit to be) a Soma-cow. For when Indra and Vishnu divided a thousand (cows) into three parts, there was one left, and her they caused to propagate herself in three kinds; and hence, even now, if any one were to divide a thousand by three, one would remain over.