Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.2.9

1.3.2.9

atha yadaṣṭau kṛtva upabhṛti gṛhṇāti | candobhyastadgṛhṇātyanuyājebhyo hi tadgṛhṇāti candāṃsi hyanuyājāstattadanādiśyājyasyaiva rūpeṇa gṛhṇātyajāmitāyai jāmi ha kuryādyadgāyatryai tvā triṣṭubhe tveti gṛhṇīyāttasmādanādiśyājyasyaiva rūpeṇa gṛhṇāti

When, on the other hand, he takes eight times (with the sruva) in the upabhrit, he takes it for the metres, since it is for the after-offerings that he takes it; and the after-offerings are the metres: all this he takes in the form of butter without making any announcement, in order to avoid sameness; for were he to take it with the formulas 'For the Gâyatrî--thee!' 'For the Trishtubh--thee!' he would commit a repetition: he therefore takes it in the form of butter without making any announcement.