Shiva Purana
रात्रावुदण्मुखः कुर्याद्देवकार्यं सदैव हि । शिवार्चनं सदाप्येवं शुचिः कुर्यादुदण्मुखः ५३ ।
Divine rites shall always be performed facing the north in the night. Similarly Śiva’s worship shall always be performed facing the north, not the east.
english translation
rAtrAvudaNmukhaH kuryAddevakAryaM sadaiva hi । zivArcanaM sadApyevaM zuciH kuryAdudaNmukhaH 53 ।
hk transliteration by Sanscript