Aitareyabrahmana 5.7

5.7

Here they praise in the Mahānāmnī [verses] with the Śākvara Sāman, on a Rathantara day, on the fifth day: [it is] the form of the fifth day.
Indra indeed created his great self by means of these; therefore [they are called] the Mahānāmnīs.
Moreover, these worlds are the Mahānāmnīs; these [worlds] are great.
Prajāpati, having created these worlds, was capable of (aśaknot) all this, whatever there is.
Since Prajāpati, having created these worlds, was capable of all this, whatever there is,
therefore they became the Śakvarīs; that is the Śakvarī-nature of the Śakvarīs.
Rising upwards, he created the borders.
Since, rising upwards, he created the borders, therefore they became the Sīmans; that is the Sīman-nature of the Sīmans.
'From the sweet, thus, of the central one' [ṚV 8.89.1], 'To us with the two bay steeds, the pressed [soma]' [ṚV 8.89.2], 'All [songs] have magnified Indra' [ṚV 8.89.3]—this is the anurūpa, having the word 'bull' (vṛṣaṇvān), having the word 'speckled' (pṛśnimān), having the word 'sweet' (madvān), having the word 'increase' (vṛdhanvān), on the fifth day: [it is] the form of the fifth day.
'Which has won' [ṚV 8.89.7] is the constant dhāyyā.
With 'We call upon thee, O hero' [ṚV 7.32.22], he turns back along the womb of the Rathantara, for this day is a Rathantara [day] in its abode.
'By no means, though priests [call upon thee]' [ṚV 7.32.20] is the Sāmapragātha, possessing repetitions (adhyāsavān), having the form of cattle, on the fifth day: [it is] the form of the fifth day.
'That swift courser, urged by the gods' [ṚV 10.178.1] is the constant Tārkṣya hymn.