Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 35
Sanskrit:
तप:सारमयं त्वाष्ट्रं वृत्रो येन विपाटित: ।अन्ये चापि बलोपेता: सर्वास्त्रैरक्षतत्वच: ॥ ३५ ॥
ITRANS:
tapaḥ-sāramayaṁ tvāṣṭraṁvṛtro yena vipāṭitaḥanye cāpi balopetāḥsarvāstrair akṣata-tvacaḥ
Translation:
Vṛtrāsura was the essence of the austerities undergone by Tvaṣṭā, yet the thunderbolt killed him. Indeed, not only he but also many other stalwart heroes, whose very skin could not be injured even by all kinds of weapons, were killed by the same thunderbolt.