Śāṅkhāyana (Kauṣītaki) Āraṇyaka 4.5

4.5

Now next, the restraint taught by Pratardana; they call it the 'inner Agnihotra'. As long, indeed, as a person speaks, so long is he unable to breathe; he then offers breath into speech. As long, indeed, as a person breathes, so long is he unable to speak; he then offers speech into breath. These two endless, immortal oblations he offers continuously, whether waking or sleeping. Now, whatever other oblations there are, they have an end, for they consist of ritual action. Knowing this, indeed, the ancients did not offer the [external] Agnihotra.