Skandapurana 1.3.15.185

1.3.15.185

vṛtrāsuro daityapatiśca śakra te samādhinā parameṇaiva jayyaḥ..
niśamya vākyaṃ parameṣṭhino hariḥ sasmāra devaṃ vṛṣabhadhvajaṃ tadā.. 17.185 ..

He is unconquerable by severe austerity; therefore, conquer him through austerity! / Vṛtrāsura, the lord of the Daityas, O Śakra, is to be conquered by you only through supreme meditation.' / Hearing the word of the Supreme Lord, Indra then remembered the god whose banner bears the bull.