Shatapatha Brahmana 5.5.4.14

5.5.4.14

tadyadetayā rājasūyayājī yajate | savānvā eṣa yajñakratūnavarunddhe sarvā iṣṭīrapi darvihomānyo rājasūyena yajate devasṛṣṭā vā eṣeṣṭiryatsautrāmaṇyanayā me 'pīṣṭamasadanayāpi sūyā iti tasmādvā etayā rājasūyayājī yajate

And as to why the performer of the Râgasûya performs this offering. He who performs the Râgasûya assuredly gains for himself all sacrificial rites, all offerings, even the spoonful-oblations; and instituted by the gods indeed is this offering, the Sautrâmanî: 'May offering be made by me with this one also! may I be consecrated by this one also!' thus (he thinks, and) therefore the performer of the Râgasûya performs this offering.