Shatapatha Brahmana 13.1.7

Pragâpati desired, 'Might I perform a horse-sacrifice?' He toiled and practised fervid devotion. From the body of him, when wearied and heated, the deities departed in a sevenfold way: therefrom the Dîkshâ (initiation) was produced. He perceived those Vaisvadeva (oblations). He offered them, and by means of them he gained the Dîkshâ: and when the Sacrificer offers the Vaisvadeva (oblations) it is the Dîkshâ he thereby gains. Day after day he offers them: day after day he thus gains the Dîkshâ. Seven of them he offers; for seven were those deities that departed (from Pragâpati); it is by means of them that he (the priest) gains the Dîkshâ for him.

But, indeed, the vital airs depart from those who exceed (the duration of) the Dîkshâ. For seven days they observe it; for there are seven (outlets of) vital airs in the head, and the Dîkshâ is the vital airs: it is by means of the vital airs he gains the Dîkshâ, the vital airs, for him. He makes offering by dividing (each) deity into three parts; for the gods are of three orders, and of three orders are these worlds: he thus establishes himself in these worlds in prosperity and vital power.

They amount to one and twenty (single invocations and oblations),--there are twelve months, five seasons, these three worlds, and yonder sun as the twenty-first,--that is the divine ruling-power, that is the glory: that supreme lordship, that summit of the fallow one (the Sun), that realm of light he attains.

Thirty Audgrabhanas he offers,--of thirty syllables the Virâg (metre) consists, and the Virâg means all food: thus (he offers) for the obtainment of all food. Four Audgrabhanas he offers (on each day), and three Vaisvadevas;--they amount to seven; for there are seven vital airs of the head, and the Dîkshâ is the vital airs: by means of the vital airs he thus gains the Dîkshâ, the vital airs, for him. A full (-spoon)-oblation he offers last for the sake of invigoration and union.