Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.1.1

4.2.1.1

cakṣuṣī ha vā asya śukrāmanthinau | tadvā eṣa eva śukro ya eṣa tapati tadyadeṣa etattapati tenaiṣa śukraścandramā eva manthī

The Sukra and Manthin (grahas), forsooth, are his eyes. Now the Sukra, indeed, is he that burns yonder (the sun); and because it burns there, therefore it is (called) Sukra ('bright'). And the Manthin, indeed, is the moon.