Śatapatha Brāhmaṇa ("of a hundred paths") 7.2.2.5

7.2.2.5

yunakta sīrā vi yugā tanudhvamiti | yuñjanti hi sīraṃ vi yugāni tanvanti kṛte yonau vapateha bījamiti bījāya vā eṣā yoniṣkriyate yatsītā yathā ha vā ayonau retaḥ siñcedevaṃ tadyadakṛṣṭe vapati girā ca śruṣṭiḥ sabharā asanna iti vāgvai gīrannaṃ śruṣṭirnedīya itsṛṇyaḥ pakvameyāditi yadā vā annam pacyate 'tha tatsṛṇyopacaranti dvābhyāṃ yunakti gāyatryā ca triṣṭubhā ca tasyokto bandhuḥ

'Yoke ye the ploughs, and stretch across the yokes!'--they indeed yoke the plough, and stretch the yokes across;--'into the ready womb here cast ye the seed!' it is for the seed that that womb, the furrow, is made; and if one casts (seed) into unploughed (ground), it is just as if one were to shed seed elsewhere than into the womb. 'And plentiful yield be there through our song!'--the song is speech, and yield means food;--'and let the ripe crop go anigh the sickle!' for when food gets ripe, people approach it with the sickle. With two (verses) he yokes, a Gâyatrî and a Trishtubh one: the significance of this has been explained.