Shatapatha Brahmana 5.1.5.5

5.1.5.5

atha saptadaśa dundubhīnanuvedyantaṃ samminvanti | pratīca āgnīdhrātprajāpatiṃ vā eṣa ujjayati yo vājapeyena yajate vāgvai prajāpatireṣā vai paramā vāgyā saptadaśānāṃ dundubhīnām paramāmevaitadvāca paramam prajāpatimujjayati saptadaśa bhavanti saptadaśo vai prajāpatistatprajāpatimujjayati

And if a Râganya sacrifices,--with, 'At the impulse of the divine Savitri, of true impulsion, I have ascended unto the highest heaven of Indra!' for Indra is the Kshatra, and the Râganya is the Kshatra.