Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.1.17

3.7.1.17

athaivamabhipadya vācayati | viṣṇoḥ karmāṇi paśyata yato vratāni paspaśe indrasya yujyuḥ sakheti vajraṃ vā eṣa prāhārṣīdyo yūpamudaśiśriyadviṣṇorvijitim paśyatetyevaitadāha yadāha viṣṇoḥ karmāṇi paśyata yato vratāni paśpaśe indrasya yujyaḥ sakhetīndro vai yajñasya devatā vaiṣṇavo yūpastaṃ sendraṃ karoti tasmādāhendrasya yujyaḥ sakheti

Thereupon he makes (the sacrificer) say while touching it (Vâg. S. VI, 4; Rig-veda I, 22, 19), 'See ye the deeds of Vishnu, whereby he beheld the sacred ordinances, Indra's allied friend!' For he who has set up the sacrificial stake has hurled the thunderbolt: 'See ye Vishnu's conquest!' he means to say when he says, 'See ye the deeds of Vishnu, whereby he beheld the sacred ordinances, Indra's allied friend.' Indra, forsooth, is the deity of the sacrifice, and the sacrificial stake belongs to Vishnu; he thereby connects it with Indra; therefore he says, 'Indra's allied friend.'