Shatapatha Brahmana 13.6.1.4

13.6.1.4

ekādaśāgniṣomīyāḥ paśava upavasathe teṣāṃ samānaṃ karmaikādaśa yūpā ekādaśākṣarā triṣṭubvajrastriṣṭubvīryaṃ triṣṭubvajreṇaivaitadvīryeṇa yajamānaḥ purastātpāpmānamapahate

On the Upavasatha (day) there are eleven victims sacred to Agni and Soma: the performance for these is one and the same. There are eleven stakes,--the Trishtubh (verse) consists of eleven syllables, and the Trishtubh is a thunderbolt, and the Trishtubh is vigour: with the thunderbolt, with vigour, the Sacrificer thus from the first repels evil.