Shiva Purana
ताभ्यां तपो बलाद्दत्तं ब्रह्मणा परमेष्ठिना ॥ अवध्यत्वं जगत्यस्मिन्पुरुषैरखिलैरपि ॥ २५ ॥
As a result of the power of their penance Brahmā granted them the boon that they would not be killed by any man in the universe.
english translation
tAbhyAM tapo balAddattaM brahmaNA parameSThinA ॥ avadhyatvaM jagatyasminpuruSairakhilairapi ॥ 25 ॥
hk transliteration by Sanscript