Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.2.6

1.2.2.6

so 'sāvājyamadhiśrayati | iṣe tveti vṛṣṭyai tadāha yadāheṣe tveti tatpunarudvāsayatyūrje tveti yo vṛṣṭādūrgraso jāyate tasmai tadāha

That one (the Âgnîdhra) puts the butter on, with the text (Vâg. S. I, 22): 'For sap--thee!' When he says 'for sap thee!' he says it for the sake of rain; therefore he takes it off again, with the text (Vâg. S. I, 30): 'For juice--thee!' What juice is derived (by the plants) from the rain, for that he says this.