visnor visvatmanah prabhoh
vahanatve vrtas tasya
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Purañjaya agreed to kill all the demons, on the condition that Indra would be his carrier. Because of pride, Indra could not accept this proposal, but later, by the order of the Supreme Lord, Visnu, Indra did accept it and became a great bull carrier for Purañjaya.