Verse 1.3.4

या वाक्सर्क्तस्मादप्राणन्ननपानन्नृचमभिव्याहरति यर्क्तत्साम तस्मादप्राणन्ननपानन्साम गायति यत्साम स उद्गीथस्तस्मादप्राणन्ननपानन्नुद्गायति ॥ १.३.४ ॥

yā vāksarktasmādaprāṇannanapānannṛcamabhivyāharati yarktatsāma tasmādaprāṇannanapānansāma gāyati yatsāma sa udgīthastasmādaprāṇannanapānannudgāyati || 1.3.4 ||

4. Whatever is vāk [speech] is also the Ṛk [part of the Ṛg Veda]. This is why a person stops breathing in and breathing out when reciting the Ṛk mantras. Whatever is the Ṛk is also the Sāma. This is why one recites the Sāma without breathing in or breathing out. That which is the Sāma is also the udgītha. This is why when one sings the udgītha one stops both breathing in and breathing out.

Word-for-word explanation:

Yā vāk sā ṛk, that which is speech is Ṛk; tasmāt, because [they are one]; aprāṇan anapānan, breathing in and out are suspended; ṛcam, the Ṛg Veda; abhivyāharati, one recites; yā ṛk tat sāma, that which is the Ṛk is also the Sāma; tasmāt, because [they are one and the same]; aprāṇan anapānan sāma gāyati, one suspends breathing in and out when singing the Sāma; yat sāma saḥ udgīthaḥ, that which is the Sāma is the udgītha; tasmāt aprāṇan anapānan udgāyati, because one sings the udgītha by suspending both breathing in and breathing out.

Commentary:

The Ṛk is a collection of words, and the Sāma is based on the Ṛk. Again, the Sāma and the udgītha are the same. To recite any of these, or even to speak, you have to resort to vyāna—that is, you must hold your breath.