Chandogya Upanishad 2.12

abhimanthati sa hiṅkāraḥ dhūmo jāyate sa prastāvaḥ jvalati sa udgīthaḥ aṅgārā bhavanti sa pratihāraḥ upaśāmyati tan nidhanam saṃśāmyati tan nidhanam etad rathaṃtaram agnau protam

The hinkâra is, he rubs (the fire-stick); the prastâva, smoke rises; the udgîtha, it burns; the pratihâra, there are glowing coals; the nidhana, it goes down; the nidhana, it is gone out. This is the Rathantara Sâman as interwoven in fire.

sa ya evam etad rathaṃtaram agnau protaṃ veda brahmavarcasyannādo bhavati sarvam āyur eti jyog jīvati mahān prajayā paśubhir bhavati mahān kīrtyā na pratyaṅṅ agnim ācāmen na niṣṭhīvet tad vratam

He who thus knows this Rathantara interwoven in fire, becomes radiant and strong. He reaches the full life, he lives long, becomes great with children and cattle, great by fame. The rule is, 'Do not rinse the mouth or spit before the fire.'