Matsyapurana 47.82

47.82

evamukto 'bravīddevo vrataṃ tvaṃ cara bhārgava
pūrṇaṃ varṣasahasraṃ tu kaṇadhūmamavākśirāḥ
yadi pāsyasi bhadraṃ te tato mantrānavāpsyasi //

Mahâdeva said :—“Bhârgava ! You shall acquire those spells after practising austerities with your head suspended down and imbibing the smoke of chaff for a thousand years. If thou succeedest in this, thou shalt obtain the mantras.”—82. Then Śukra, accepting the injunctions of the Lord Śiva, touched His feet and said, "Sire! In obedience to your will, I start to observe the penances dictated by you."—83.