O brāhmaṇa, during the seventh reign of Manu, the current age, Lord Indra came to know of Mārkaṇḍeya’s austerities and became fearful of his growing mystic potency. Thus he tried to impede the sage’s penance.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
एतत्पुरन्दरो ज्ञात्वा सप्तमेऽस्मिन् किलान्तरे । तपोविशङ्कितो ब्रह्मन्नारेभे तद्विघातनम्
etatpurandaro jñātvā saptame'smin kilāntare | tapoviśaṅkito brahmannārebhe tadvighātanam
SB 12.8.15