Jaiminīya (Talavakāra) Brāhmaṇa 7.25.p6

7.25.p6

tadyatpurastādyauktāśvaṃ bhavati vāsiṣṭhamupariṣṭādṛṣisāmanī brahmaṇa eva sa

Now, because the Yauktāśva is in the beginning, and the Vāsiṣṭha later, the two seer-sāmans are for the counterbalancing of the brahman,