Shiva Purana
अष्टाभ्यां च चतुर्दश्यां पौर्णमास्यां विशेषतः ॥ एकादश्यां च विधिवदुपवासोममार्चनम् ॥ २३ ॥
On the eighth and fourteenth days of the month, especially on the full moon day, On the Ekadashi one should fast and worship the Soma according to the rituals.
english translation
aSTAbhyAM ca caturdazyAM paurNamAsyAM vizeSataH ॥ ekAdazyAM ca vidhivadupavAsomamArcanam ॥ 23 ॥
hk transliteration by Sanscript