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, Viṣṇu, Indra did accept it and became a great bull carrier for Purañjaya.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
वचनाद्देवदेवस्य विष्णोर्विश्वात्मनः प्रभोः । वाहनत्वे वृतस्तस्य बभूवेन्द्रो महावृषः
vacanāddevadevasya viṣṇorviśvātmanaḥ prabhoḥ | vāhanatve vṛtastasya babhūvendro mahāvṛṣaḥ
SB 9.6.14