Verse 2.21.1

त्रयी विद्या हिंकारस्त्रय इमे लोकाः स प्रस्तावोऽग्निर्वायुरादित्यः स उद्गीथो नक्षत्राणि वयांसि मरीचयः स प्रतिहारः सर्पा गन्धर्वाः पितरस्तन्निधनमेतत्साम सर्वस्मिन्प्रोतम् ॥ २.२१.१ ॥

trayī vidyā hiṃkārastraya ime lokāḥ sa prastāvo'gnirvāyurādityaḥ sa udgītho nakṣatrāṇi vayāṃsi marīcayaḥ sa pratihāraḥ sarpā gandharvāḥ pitarastannidhanametatsāma sarvasminprotam || 2.21.1 ||

1. The three vidyās [the Ṛk, the Yajuḥ, and the Sāma] are together the hiṃkāra; these three worlds [the earth, the space between the earth and heaven, and heaven] are together the prastāva; fire, air, and the sun are together the udgītha; the stars, the birds, and the rays are together the pratihāra; serpents, gandharvas, and the ancestors are together the nidhana. This Sāma resides in everything.

Word-for-word explanation:

Trayī vidyā hiṃkāraḥ, the three Vedas [the Ṛk, the Yajuḥ, and the Sāma] are the hiṃkāra; trayaḥ ime lokāḥ, these three worlds [bhūḥ, bhuvaḥ, svaḥ]; saḥ prastāvaḥ, [together] that is the prastāva; agniḥ vāyuḥ ādityaḥ, fire, air, and the sun; saḥ udgīthaḥ, [together] that is the udgītha; nakṣatrāṇi vayāṃsi marīcayaḥ, the stars, the birds, and the rays; saḥ pratihāraḥ, [together] that is the pratihāra; sarpaḥ gandharvāḥ pitaraḥ, serpents, gandharvas [celestial musicians], and the ancestors; tat nidhanam, [together] that is the nidhana; etat sāma sarvasmin protam, this Sāma rests in everything.

Commentary:

According to the scriptures the Ṛg Veda comes from fire, the Yajur Veda comes from air, and the Sāma Veda comes from the sun. In the previous section the worship of the Sāma as fire, air, etc., was discussed. Now the worship of the Sāma as the three Vedas is taken up.

Knowledge of the three Vedas may be regarded as the hiṃkāra, for you have to use the hiṃkāra before you start doing any worship. After the hiṃkāra comes the prastāva. Similarly, after the three Vedas come the three worlds, so they are said to be the prastāva. Fire, air, and the sun are like the udgītha, for among all things they are of a higher order. The stars, the birds, and the rays come and go, so they are like the pratihāra. The serpents, the gandharvas, and

The word Sāma does not refer to any specific Sāma. It refers to anything bearing that name. The word is meant according to the context, just as oblations are offered according to the deities for whom they are meant.