Shiva Purana
चतुर्दशाब्दं कर्तव्यं शिवरात्रिव्रतं शुभम्॥ एकभक्तं त्रयोदश्यां चतुर्दश्यामुपोषणम्॥ ३॥
The auspicious Śivarātrivrata shall be performed for fourteen years. On the Trayodaśī day only one meal shall be taken and a complete fast shall be observed on the Caturdaśī day.
english translation
caturdazAbdaM kartavyaM zivarAtrivrataM zubham॥ ekabhaktaM trayodazyAM caturdazyAmupoSaNam॥ 3॥
hk transliteration by Sanscript