Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.3.25

1.3.25

tasya haitasya sāmno yaḥ svaṃ veda bhavati hāsya svaṃ tasya vai svara eva svaṃ tasmādārtvijyaṃ kariṣyanvāci svaramiccheta tayā vācā svarasaṃpannayārtvijyaṃ kuryāttasmādyajñe svaravantaṃ didṛkṣanta eva | atho yasya svaṃ bhavati bhavati hāsya svaṃ ya evametatsāmnaḥ svaṃ veda

He who knows what is the property of this Sâman, obtains property. Now verily its property is tone only. Therefore let a priest, who is going to perform the sacrificial work of a Sama-singer, desire that his voice may have a good tone, and let him perform the sacrifice with a voice that is in good tone. Therefore people (who want a priest) for a sacrifice, look out for one who possesses a good voice, as for one who possesses property. He who thus knows what is the property of that Sâman, obtains property.