इयमेवर्गग्निः साम तदेतदेतस्यामृच्यध्यूढ़ं साम तस्मादृच्यध्यूढंसाम गीयत इयमेव साग्निरमस्तत्साम ॥ १.६.१ ॥
iyamevargagniḥ sāma tadetadetasyāmṛcyadhyūḍha़ṃ sāma tasmādṛcyadhyūḍhaṃsāma gīyata iyameva sāgniramastatsāma || 1.6.1 ||
1. This earth is like the Ṛg Veda, and fire is like the Sāma Veda. The Sāma is based on the Ṛg Veda, and this is why the Sāṃa is sung based on the Ṛg Veda. The earth is sā and fire is ama. This sā and ama together make Sāma.
Iyam, this [earth]; eva ṛk, is like the Ṛg Veda; agniḥ sāma, fire is like the Sāma Veda; tat etat sāma etasyām ṛci adhyūḍham, the Sāma Veda is based on the Ṛg Veda; tasmāt, therefore; ṛci adhyūḍham sāma gīyate, the Sāma Veda is sung based on the Ṛg Veda; iyam eva sā, this [earth] is the sā [of Sāma]; agniḥ amaḥ, fire is ama [of Sāma]; tat sāma, that [if joined together] is Sāma [i.e., sā + ama].
The Upaniṣad says to treat the earth as the Ṛg Veda and fire as the Sāma Veda. But how are they so related? The Sāma is based on the Ṛg Veda, and fire is based on the earth. Because of this relationship between them, they are thought of as identical.