Śatapatha Brāhmaṇa ("of a hundred paths") 11.5.2.1

11.5.2.1

prajāpatirha cāturmāsyairātmānaṃ vidadhe sa imameva dakṣiṇam bāhuṃ vaiśvadevaṃ havirakuruta tasyāyamevāṅguṣṭha āgneyaṃ haviridaṃ saumyamidaṃ sāvitraṃ

By means of the Seasonal sacrifices, Pragâpati fashioned for himself a body. The sacrificial food for the Vaisvadeva sacrifice he made to be this right arm of his; the oblation to Agni thereof this thumb; that to Soma this (fore-finger); and that to Savitri this (middle finger).