Shiva Purana
पूर्वसत्काररूपेण महापुण्येन शंकरः ॥ हंसरूपं विधायैव ताभ्यां सुखमदात्प्रभुः ॥ ३६ ॥
It was due to the merit accruing from hospitality that lord Śiva gave them pleasure after assuming the form of a swan.
english translation
pUrvasatkArarUpeNa mahApuNyena zaMkaraH ॥ haMsarUpaM vidhAyaiva tAbhyAM sukhamadAtprabhuH ॥ 36 ॥
hk transliteration by Sanscript