Manava Dharmashastra 6.71

6.71

dhātavaḥ suvarṇādayaḥ, teṣāṃ dhmāyamānānāṃ suvarṇamevāvaśiṣyate | tathendriyāṇāṃ viṣayadarśane yau prītiparitāpau jāyete tayoryatpāpaṃ tasya dāhaḥ prāṇanirodhāt | prītiparitāpotpattirmumukṣorniṣiddhā | sā tu śarīriṇaḥ tyaktasaṃgasyāpi yādṛcchikarūpaśabdādyupanatau kayācinmātrayā vastusāmarthyena niyatendriyasyāpyupajāyate | atastaddoṣanivṛttyarthāḥ prāṇāyāmāḥ |

Just as the impurities of metallic ores are consumed when they are blasted, even so are the taints of the senses consumed through the suspension of breath.—(71)

Medhātithi’s commentary (manubhāṣya):
When the ‘metallic ores,’ of gold for instance, are blasted in a furnace, what is left behind is pure gold; similarly when the senses apprehend their objects, the man feels joys and sorrows, and these are productive of sin; this sin is consumed through the suspension of breath.

For the man seeking Liberation, indulging in joys and griefs has been forbidden.

But even in a man who has given up all attachment, and has his organs under his control, these are bound to appear, in howsoever small a degree, through the sheer nature of things, whenever by chance various kinds of colour, sound &c. become presented before him. And it is for the removal of the taints due to these that breath-suspensions have to be practised.—(71)

This verse is quoted in Mitākṣarā (on 3.62);—and in Vidhānapārijāta (II. p. 176).

Comparative notes by various authors
Vaśiṣṭha (25.6).—‘Through the inhibition of breath air is generated; through air is produced fire; then through heat water is formed; hence one becomes internally purified by these three.’

Baudhāyana (4.1.24).—(Same as above from Vaśiṣṭha.)