Shiva Purana
एतस्मिन्नन्तरे शंभुर्विष्ण्वाद्यैः प्रार्थितः प्रभुः ॥ परीक्षितुं च तद्भक्तिं शक्ररूपोऽभवत्तदा॥ १९॥
In the meantime, requested by Viṣṇu aud others, Śiva assumed the form of Indra in order to test his devotion.
english translation
etasminnantare zaMbhurviSNvAdyaiH prArthitaH prabhuH ॥ parIkSituM ca tadbhaktiM zakrarUpo'bhavattadA॥ 19॥
hk transliteration by Sanscript