Shatapatha Brahmana 9.5.1.33

9.5.1.33

yadvevaikādaśa | ekādaśākṣarā vai triṣṭuptraiṣṭhubha indra indro yajñasyātmendro devatā tadya eva yajñasyātmā yā devatā tasminnevaitadyajñamantataḥ pratiṣṭhāpayati

And, again, as to why there are eleven,--the Trishtubh consists of eleven syllables, and Indra is of trishtubh nature, Indra is the self (soul) of the sacrifice, Indra is the deity: he thus finally establishes the sacrifice in him who is the self, the deity of the sacrifice.