Shiva Purana
आस्थाय विपुलं रूपं वाराहममितं विधिः ॥ पृथिव्युद्धरणार्थाय प्रविवेश रसातलम् ॥ २९ ॥
The method is immeasurable, assuming the vast form of a boar. He entered the bottom of the ocean to lift the earth.
english translation
AsthAya vipulaM rUpaM vArAhamamitaM vidhiH ॥ pRthivyuddharaNArthAya praviveza rasAtalam ॥ 29 ॥
hk transliteration by Sanscript