Shiva Purana
तपसा लभ्यते राज्यं स च शक्रस्सुरेश्वरः ॥ तपसाऽपालयत्सर्वमहन्यहनि वृत्रहा ॥ ४८॥
Kingdoms are achieved through penance. Indra, the lord of gods and the slayer of Vṛtra, protects everything day by day through penance.
english translation
tapasA labhyate rAjyaM sa ca zakrassurezvaraH ॥ tapasA'pAlayatsarvamahanyahani vRtrahA ॥ 48॥
hk transliteration by Sanscript