Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.1.25

3.9.1.25

tānyatra nividhyanti | āgneyameva prathamaṃ dakṣiṇārdhyaṃ nividhyantyathetarānudīco 'tinīya yathāpūrvam

And as to why it comes after that for Sarasvatî,--Sarasvatî is speech, and Soma is food: he who is incomplete by (having only) speech, now becomes indeed an eater of food.