Shatapatha Brahmana 3.1.4.6

3.1.4.6

athāto homasyaiva | ākutyai prayuje 'gnaye svāhetyā vā agre kuvate yajeyeti tadyadevātra yajñasya tadevaitatsambhṛtyātmankurute

Now then of the oblation (Vâg. S. IV, 7): 'To the Purpose, to the Impulse, to Agni, hail!' At the outset he indeed purposes to sacrifice. What part of the sacrifice (is contained) in this (first libation), that he now collects and makes his own.