Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.2.41

2.5.2.41

athādhvaryurevāha brahmanprasthāsyāmi | samidhamādhāyāgnimagnītsammṛḍḍhīti sa srucorevādhvaryuḥ pṛṣadājyaṃ vyānayate 'tha yadi pratiprasthātuḥ pṛṣadājyam bhavati tatsa dvedhā vyānayata uto tatra pṛṣadājyaṃ na bhavati sa yadevopabhṛtyājyaṃ tatsa dvedhā vyānayate tā ubhāvevātikrāmato 'tikramyāśrāvyādhvaryurevāha devānyajeti yaja-yajeti caturthe-caturthe 'nuyāje samānayamānau navabhiranuyājaiścaratastadyannavaprayājam bhavati navānuyājaṃ tadubhayata evaitadvaruṇapāśātprajāḥ pramuñcatītaścordhvā itaścāvācīstasmānnavaprayājam bhavati navānuyājam

Thereupon the Adhvaryu says, 'O Brahman, shall I step forward?' Having put on the (remaining) kindling-stick he says, Agnîdh, trim the fire! He, the Adhvaryu, then pours the clotted butter (in the prishadâgya-upabhrit) into the two spoons (the guhû and upabhrit); and the Pratiprasthâtri also, if he have any clotted butter, divides it into two parts and pours it (into the two spoons); but if there is no clotted butter, he divides the butter in the upabhrit in two parts and pours them out separately. Then both step across (to the south side of the fires). The Adhvaryu, having stepped across and called for the 'Sraushat,' says (to the Hotri), 'Pronounce the offering-formula to the gods!' and, 'Pronounce the offering-formula!' at each (subsequent after-offering). Thus they both perform the nine after-offerings, pouring together (the butter from the spoons) at the (or at every) fourth after-offering. The reason why there are nine fore-offerings and nine after-offerings, is that he thereby delivers the creatures both times from Varuna's noose,--by the former (he delivers) the upright and by the latter those looking to the ground: for this reason there are nine fore-offerings and nine after-offerings.