Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 14
Sanskrit:
वचनाद् देवदेवस्य विष्णोर्विश्वात्मन: प्रभो: ।वाहनत्वे वृतस्तस्य बभूवेन्द्रो महावृष: ॥ १४ ॥
ITRANS:
vacanād deva-devasyaviṣṇor viśvātmanaḥ prabhoḥvāhanatve vṛtas tasyababhūvendro mahā-vṛṣaḥ
Translation:
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.