Śatapatha Brāhmaṇa ("of a hundred paths") 5.3.3.4

5.3.3.4

atha somāya vanaspataye | śyāmākaṃ carum nirvapati tadenaṃ soma eva vanaspatiroṣadhibhyaḥ suvatyatha yacyāmāko bhavatyete vai somasyauṣadhīnāṃ pratyakṣatamāṃ yacyāmākāstasmācyāmāko bhavati

For Soma Vanaspati (the wood-lord or tree) he then prepares a pap of syâmâka millet: thereby Soma, the wood-lord, quickens him for the plants. And as to its being prepared of syâmâka,--they, the syâmâkas among plants doubtless are most manifestly Soma's own: therefore it is prepared of syâmâka grain.