Shiva Purana

siva.sh/siva-purana/rudra-samhita/197/27

तौ निपात्य महादैत्यावकार्यकरणोद्यतौ ।। ततः परिणतिं यातो लिंगरूपेण कंदुकः।।२७।।

sanskrit

As they had attempted to do an evil action, the ball changed itself into the phallic image.

english translation

tau nipAtya mahAdaityAvakAryakaraNodyatau || tataH pariNatiM yAto liMgarUpeNa kaMdukaH||27||

hk transliteration by Sanscript