Shiva Purana
यदैताभ्यां परप्रीत्या क्रियते सुतपः पुनः ॥ एतादृशं शुभं चैतत्तपो दुष्करमुत्तमम् ॥ ३३ ॥
When these two again perform great austerities out of love for others This is such an auspicious and excellent difficult austerity.
english translation
yadaitAbhyAM paraprItyA kriyate sutapaH punaH ॥ etAdRzaM zubhaM caitattapo duSkaramuttamam ॥ 33 ॥
hk transliteration by Sanscript