Shiva Purana
इत्येवं कुर्वतस्तस्य जलं बिल्वदलानि च ॥ पतितानि ह्यधस्तत्र शिवलिंगमभूत्ततः ॥ १९॥
While he was doing this he offered him water and bilva leaves They fell to the ground and then a Shiva Linga appeared.
english translation
ityevaM kurvatastasya jalaM bilvadalAni ca ॥ patitAni hyadhastatra zivaliMgamabhUttataH ॥ 19॥
hk transliteration by Sanscript