Bhagavatapurana 6.13.15

6.13.15

sa āvasat puṣkaranālatantūn alabdhabhogo yadihāgnidūtaḥ /
varṣāṇi sāhasramalakṣito 'ntaḥ sañcintayan brahmavadhādvimokṣam //

15. He (Indra) dwelt in the fibres of the lotus-stalk and found nothing to subsist upon, since his messenger Agni could not enter there; thus he was in there for a thousand years unperceived, pondering how to cleanse himself of the sin of killing the brahmin.