Shiva Purana
सोऽम्बरीषो नृपवरः सप्तद्वीपरसापतिः ॥ नियमं हि चकारासावेकादश्या व्रते दृढम् ॥ ३० ॥
The king Ambarīṣa was the lord of the earth consisting of seven continents. He observed the rite of Ekādaśī regularly with steady restraint.
english translation
so'mbarISo nRpavaraH saptadvIparasApatiH ॥ niyamaM hi cakArAsAvekAdazyA vrate dRDham ॥ 30 ॥
hk transliteration by Sanscript